PSTN Calls from mobile – wrong “from” header in hybrid voice

I recently discovered a minor issue at a customer site with Cloud Connector Edition deployed connecting SKYPE for Business online in Office365 to onprem PSTN (SIP Trunk)

The issue has been replicated and confirmed by Office365 engineer team – there is at the moment no solid workaround – but issue is being looked in to.

The problem occurs when you dial out to PSTN from the Skype for Business mobile client on Android/ iOS / WinPhone. The FROM header that is sent in the invite from the Mediation server component in the CCE to the PSTN Gateway  is missing the LineUri in the FROM field – and also the ;user=phone is also missing – instead the users sip-address is sent.
Meaning that From header for my test user is

Where it should be
“FROM: sip:+45xxxxxxxx@PSTNGatewayFQDN;user=phone”

So what does this mean ? – Well 2 things.

When sending this info to a SIP Trunk provider or ISDN – they cannot forward the user name as a valid E.164 number, and also on many platforms calls will be rejected  because the FQDN is not contained in the FROM field – so unless the provider has knowledge of the Vanity domain call will fail – because the property ;user=phone is missing
The latter, can be fixed easily by the provider -so they will accept the call – bu the primary issue is that calls originating in the mobile client will show up a callee as “Private number” or unknown.

Only workaround would be 1:1 regex replacement in the gateway – and unless the customer has only a few employees, this is not a feasible solution in my opinion.

By the way – a small “note from the field” always include Wireshark and enable Telnet in the BaseVM before running install-ccinstance 🙂 Saves a few minutes when troubleshooting issues like this.
As always Happy (cloud) SKYPE’ing.

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