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No Ringback Heard by Caller – Skype for Business & Audiocodes

This problem presented itself at one of my customers a couple of weeks ago. I thought it was an interesting case that I should share this incase you find yourself in a similar situation. The problem started withe an e-mail to me from a current client saying “Please can you help? When someone calls our main office line they don’t hear the phone ring”. To set the scene this means that when the Caller places a call to a number, they don’t hear the ringing tone in their ear. This has a few names, but for consistency with telephony naming, we will call this ringback.

A few more pointed questions later, I determined that when the Caller (Calling Party) placed the call, the Callee (Called Party) recieved ringback and when answered, the media was established and the called party could have a conversation with the calling party. This could get a bit confusing so perhaps this simple diagram will help


Skype for Business and Establishing Media Deep Dive

Following on from my blog post on the basics of SIP call setup, I wanted to build on that to incorporate how media is negotiated between clients. Again, this topic has been blogged by others and the purpose of this is to put my understanding into my own words so that I understand and if it helps you, then great!

When a user initiates a call to another endpoint to begin the conversation a SIP INVITE is sent from the calling party to their SIP Proxy which forwards that request. I covered the SIP portion of the message in my previous article here, but SIP in itself will not fully establish the call flow, it will simply connect two or more endpoints together for communication. For voice to pass between endpoints a media stream is required. Whether this is direct between clients, known as P2P (Peer to Peer), or via a media relay server such as the Skype for Business Edge Server, or Mediation Server.


Legacy PBX Number Presentation to Skype for Business Using Sonus SBCs and AD based RNL

A customer messaged me last week and asked me a question regarding number presentation to Skype for Business when users still on their legacy PBX system placed a call to a Skype for Business endpoint. The customer explained to me that when a Skype for Business user called an extension on their legacy PBX system, the caller display name showed the name of the caller. However, when a legacy PBX user called a Skype for Business endpoint, the caller display showed the extension number rather than the name of the caller, therefore not allowing the Skype for Business user to easily identify the caller.

The reason why when a Skype for Business user calls the legacy PBX extension the caller’s name is display is because out of the box, Skype for Business sends both the extension number and the name of the caller in the FROM header in the SIP INVITE to the legacy PBX. For Example:

FROM: “David Williams” <sip:+441270212000@skypefe1.domain.com:5068;user=phone>

Using SIP standards, the legacy PBX automatically interprets this and is able to display the name of the caller to the handset.

However, the legacy PBX system appeared to be configured with just extension numbers only. The system had no local directory to associate an extension with a name of the employee who has that extension supplied to them. The result of this was that when the legacy PBX sent the INVITE to Skype for Business the FROM header simply included just the extension number, like

FROM: sip:1005@legacypbx.domain.com:5060;user=phone

End user experience on a Skype for Business client is:


Skype for Business and Sonus–Remote Site Survivability (an alternative approach)

When deploying Skype for Business to medium sized organisations, the main challenges tend to be around how to ensure that remote sites adjacent to the main headquarters remain online for as long as possible in the event of an infrastructure failure. If you follow the Skype for Business architecture, you will know that the best practice is to deploy some hardware to remote sites that warrant it. Whether the requirement is down to user count, or a business critical process mandates a remote site’s availability service level agreement, this is often a battle between affordability and best practice. We Skype consultants will always peddle best practice recommendations to our customers, because we know they work, and more importantly, we know that the solution is backed by both Microsoft, and other hardware / software vendors. This results in our customer’s receiving the best possible after care service, a supported topology and upgrade path, ensuring the integrity of the complete product lifecycle is maintained.


Skype for Business and Sonus–Part 6–Getting around NAT

In Part 5 we discussed how to handle encrypted signalling and media. In this article we will be discussing how to configure the Sonus SBC to work behind a NAT firewall. If you have been following this blog series, in part 1 we discussed the recommended connectivity setup. This was to connect the SBC directly to your WAN by public IP address. However, there are some (actually majority) of customers who have a network configuration that is not optimised for this type of connectivity. In these scenarios, the SBC usually sits behind the network edge firewall and services are passed through from the outside world to the SBC using NAT.


Skype for Business and Sonus – Part 5 – Adding Encryption

In Part 4, we discussed and walked through how to connect the SBC up between the PSTN and Skype for Business using simple unencrypted signalling and media. In this article we will discuss how to create a secure connection to Skype for Business and encrypt the media traffic between them.

The first thing we must do is request and install an SSL certificate on the SBC. This can be from either a trusted SSL provider (if you are connecting to external services over TLS too) or from your internal PKI system, if you are encrypting internal traffic only.


Skype for Business and Sonus – Part 4 – Connecting it all together

In Part 3 of this series, we configured Skype for Business for enterprise voice and added the Sonus SBC to the topology as an available PSTN gateway. In this article we will discuss and walkthrough how to accept telephone calls from Skype for Business and route to the PSTN and vice versa.

Before we begin with the walkthrough guides, let’s take a moment to discuss how the SBC will handle this. As we go through this guide you will hear words like Signalling Group (SG), Transformation Table, Call Route, Media List, SIP Server table etc. Don’t be alarmed by the time this is over, you will be comfortable with these terms and what they mean and their role in the successful call flow. To begin with I will show you the call setup (signalling) workflow through the SBC from Skype for Business to the PSTN.


Skype for Business and Sonus – Part 3 – Configuring Skype for Business

In Part 2 we discussed and walked through how to configure the Sonus SBC system settings including networking, security and other system dependencies. In this article we will be configuring Skype for Business to use the Sonus SBC as an appropriate gateway to the PSTN for Enterprise Voice. To begin with we will be making the connection using unencrypted TCP connections to make it simple. TLS trunk configuration will come in a later article that will cover some common advanced configurations and settings.

As part of this article we will setup the gateway and trunk inside Skype for Business, create a simple dial plan and voice policy and assign a user a DDI so that we can test inbound and outbound calling later on. Please excuse the briefness of some topics as this article assumes a level of competency with Skype for Business.

Firstly, please make sure that you have a DNS record configured for your Sonus SBC in your internal DNS zone. This record is a simple “A” or pointer record to the SBC IP. Alternatively, an IP address of the SBC can be used instead of the FQDN for TCP connectivity.


Skype for Business and Sonus – Part 2 – Configuring the Foundations

In Part 1 we discussed the basics of where and how to install the Sonus SBC. In this article we will be discussing how to prepare the SBC 1000 for production readiness including; licencing, updating, controlling access, networking and system settings.

It is important to set solid footings in order to ensure easier administration moving forward and for proper security.


The first task I undertake is to licence the SBC. Out of the box, the device is pretty much an expensive book stop. With no licence applied, essentially all you can do is make one registration to the SBC but no calls can be placed through the device. Gather your device serial number you obtained from part 1. When you purchase an SBC the device should be accompanied by an email from Sonus that provides a link to their self-licencing portal and access code. If you do not have this email, please contact your supplier to obtain this. Once submitted your licence key will be provided to you via e-mail. This e-mail can take about 5 minutes to arrive, so don’t panic if it not sitting in your inbox immediately. The key is hash of your device serial number, the features purchased and the validity period of the licence. Copy the licence key to your clipboard and from your SBC, click on Settings tab > System > Licensing > Install New License


Skype for Business – Is CloudPBX Enterprise Voice?

With Microsoft’s latest announcement that they are bringing PSTN voice calling to the Office 365 public cloud offering and released tech preview trials in the USA recently has left the industry in a flurry of excitement, rumour and ponder. Partners are hurriedly preparing presentations, marketing material and sales pitches. Customers are beginning to plant the seed for a migration with their finance director and jostling for budget priority. Should we all be in a rush to jump to the cloud? There are a few things that might make you consider your next move. For those of you who read my blog, you may have read this article where I touched on this subject: http://skype4b.uk/2015/04/09/hybrid-or-no-hybrid-skype-for-business/.

Firstly, can we absolutely, without question call CloudPBX Enterprise Voice? Let’s evaluate.

Earlier this week Microsoft announced at WPC 2015 that they would be releasing a new Office 365 subscription plan – E5. This will replace the E4 plan. Details on what this offers you have yet to be published. However I would expect something like:

E3 Plan

  • PSTN Dial-in Conferencing
  • Skype Broadcast Meetings

E5 Plan

  • CloudPBX with PSTN Calling
  • CloudPBX with Local PSTN Calling (Possibly an optional extra)


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