I ran into an issue today where I was trying to send a call from Skype for Business (which I’ll continue to refer to as Lync here because it’s shorter) to an application via the PSTN. The telephony setup is a little strange in my lab because there are several different environments with their own distinct servers working from the same gateway, so I’m trying to call from Lync A, out through an Audiocodes Mediant 1000, then back in through the same gateway into Lync B on a configured line URI. I actually had this working from an outside line, but if I tried calling through the gateway, I’d get a 485 back on the mediation server with an error saying “Multiple users associated with the source phone number” Off to google it, and I found a blog post from three years ago describing the same issue. That I wrote… I knew this problem sounded familiar.
This time, however, I managed to figure out a solution. I think the issue goes something like this: The users on my domain had line URIs that looked like tel:+19058825000;ext=2154 (main autoattendant number plus extension). The caller ID coming from the gateway was <sip:19058825000;phone-context=PstnGateway_192.email@example.com;user=phone>;epid=0146001F41;tag=79915bb9. The front end saw this and tried to match to an account, which it appears to do without any regard for the extension parameter (since, in the Microsoft world, apparently everyone has a DID). It matched multiple users, hence the ambiguous error.
The solution, as it turns out, is to just mangle the caller ID on the gateway so that it doesn’t match any part of any user’s line URI:
This makes the from URI on a call:
Which routes just fine. I’d still probably class this as a bug, but at least now I know there’s a workaround for it. So, in three more years when I have this problem again, now I’ll find the answer when I search for the error.