Notes from the field: a=ptime causing issues returning calls to Skype for Business from Alcatel handsets

Hello Readers

I have been recently rolling out a Skype for Business Voice pilot to one of my customers and ran into a weird issue also described here https://social.microsoft.com/Forums/en-US/c97531a1-2935-4165-8d76-e9707ee84be4/488-invalid-incoming-gateway-sdp-invalid-media?forum=communicationsservertelephony. This deployment is designed to integrate with the existing Alcatel telephony system that is also managed by my organisation and the requirements are simple.

  • Be able to make a call from Skype for Business to Alcatel
  • Be able to make a call from Alcatel to Skype for Business

All calls from the Skype for Business enviromnet was successful to Alcatel endpoints, however 95% of the calls from Alcatel to Skype for Business failed.

  • Old version devices was unable to call a Skype for Business User
  • New devices was able to call a Skype for Business User
  • Phones was on the same IP subnet

After running numerous traces from Skype for Business, AmcePacket and Alcatel we noticed that the Skype for Business environment was rejecting the calls due to invalid SDP information being presented.

“488 Invalid incoming Gateway SDP”

The below SDP information is from a working call from a Alcatel endpoint to a Skype for Business Endpoint

v=0

o=OXE 1462877309 1462877309 IN IP4 10.6.1.63

s=abs

c=IN IP4 10.6.1.63

t=0 0

m=audio 49436 RTP/AVP 9 8 18 101

a=sendrecv

a=rtpmap:9 G722/8000

a=ptime:20

a=maxptime:30

a=rtpmap:8 PCMA/8000

a=ptime:20

a=maxptime:30

a=rtpmap:18 G729/8000

a=fmtp:18 annexb=no

a=ptime:20

a=maxptime:40

a=rtpmap:101 telephone-event/8000

 

The below SDP information is from a non working call from a Alcatel endpoint to a Skype for Business Endpoint

v=0

o=OXE 1462881203 1462881203 IN IP4 10.60.1.63

s=abs

c=IN IP4 10.60.1.63

t=0 0

m=audio 49198 RTP/AVP 8 18 101

a=rtpmap:8 PCMA/8000

a=rtpmap:18 G729/8000

a=fmtp:18 annexb=no

a=rtpmap:101 telephone-event/8000

a=sendrecv

a=maxptime:30

a=ptime:20

a=maxptime:40

a=ptime:30

a=maxptime:30

a=ptime:20

 

As you can see I have highlighted the ptime enteries and in the non-working call we can see 2 out of the 3 are using 20ms. We found that the 30ms entry was causing the issue and it was the Alcatal OXO providing different a=ptimes per codec.

 

Chapter 1. Solution

Telnet or Putty to your Alcatel OXO and change the following setting.

  • Go under /System/Other System Param./SIP Parameters
  • Change to False from True

 

Packetization times per codec :

  • If True , a couple of ptime/maxptime information is available for each codec.
  • If False , a single couple of ptime/max ptime information is available for all codecs.

 

Acknowledgements

Graham Cropley for assisting with this issue.

 

Regards

Andrew J. Price

Office Server and Services MVP

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