I have opened a ticket with GS and my distributor. Both have reproduced the issue
@rtl: Other forum users already posted lots of useful information. I just read all of them. If “direct IP call” works but using Avaya IP Office failed, then problem is IP Office.
The GXP21xx does not support video directly as above post mentioned, this is correct. But it can decode and display the video in very low frame rate. If UCM involved, the UCM will know the GDS involved and will pass “Call-Info” to advise GXP to decode the video stream. BUT I am sure IP Office and other IPPBX does not do this, unless stateless proxy used (like OpenSER).
You might need to contact Ayaya to see whether they can pass that info transparently.
Thank you for using Grandstream products.