If you regularly work with Sonus 1000/2000 session boarder controllers, you may often be sat there scratching your head as to why a simple inbound call from ISDN to Skype for Business won’t ring your test handset.
Before you go and make yourself another cup of coffee and spin up LX, here’s a list of common issues I frequently encounter.
ISDN Channels
Alright, so you’ve set up your signalling groups, your transformation tables are a thing of beauty and your ISDN cables are connected and green. You go to test an inbound call and all you get is a busy single. What gives?
Be sure to check that you’ve configured the correct number of ISDN channels on the SBC. Your carrier will pick a channel at random, and if you’ve configured 10 channels on the gateway but the carrier is trying to send a call down channel 15, the call will fail.
The flip side of this is ensuring you don’t over-provision the number of channels. An outbound call will fail if the SBC attempts to send a call down channel 21 if you only have 20 channels available.
Skype for Business Servers
Are you having intermittent outbound call issues, or has your Skype for Business environment recently grown? Don’t forget to add the additional mediation servers to your SIP Server Tables and Signalling groups on the SBC! If a call happens to originate from a server not added to either of these lists, it’ll fail.
Calls not releasing upon hangup
You may notice (particularly with CAS analogue lines) that when a caller hands up the phone, the line remains in release mode. Your users may not immediately notice the issue unless they go to make another call right after the previous – only to receive a busy tone.
This issue is normally caused by an incorrectly configured tone table. There’s various places around the internet to find suitable tone tables for your carrier and country that a quick Google search will locate. Be sure to update your tone tables and assign the correct tables to the correct signalling groups.
Finding an unknown called number
Lift alarms, security gates, door controls – most sites have them, and you can be pretty guaranteed that they won’t come with details around what numbers they call when a user presses the button.
So far, I’ve encountered systems that dial 10 digits, 9 digits, 5 digits and even 2 digits.
the easiest way to locate these numbers is to build a catch all rule:
Called Number (.*)   translates to Called Number 0400123456 (your mobile number).
Have someone press the button to trigger an outbound call and then use the monitoring tab to capture the phone number dialled. Once you have it, you can then build your transformation rule to capture and transform it to any number you like! (Pizza Hut, anyone?)
Sending a call to two analogue extensions at the same time
A simple request that other phone systems can manage easily. You want to route an inbound call to ring on two analogue handsets at the same time, Or maybe you have a loud ringer and an analogue handset that must ring at the same time.
The easiest way of achieving this is to use an RJ12 (or 11) splitter connected directly to the CAS port on the SBC. You can then connect up two devices to the one port, and both will ring at the same time.
You should note that the FSX cards on a 1K/2K will deliver up to 45 volts to up to 3 devices at once (REN 3).
Do you have a tip that you’d like to share? Leave a comment below!.

Skype For Business