Zoiper 5 violates RFC when answering video call request with audio only

Question

I'm using Zoiper5 5.6.6 for Linux 64bit

I've tested this with multiple calling UAs, including Yealink and Linphone. The latter sends INVITE with SDP:
m=video 44895 RTP/AVP 96 97

Zoiper responds with 200 OK and SDP:
m=video 0 RTP/AVP

Which is wrong, according to rfc3264 section 6
(Any media formats listed are ignored.  At least one MUST be present, as specified by SDP)

The second m= line for video should have at least one payload type:

m=video 0 RTP/AVP 96

Please fix.

Answer 1

Go to Zoiper5 - Settings - Video and enable "Always accept video".
This would allow Zoiper5 to answer video calls with video.
The video port is zeroed, because Zoiper drops the video part of the media with this option disabled.

Answer 2

Zoiper 5 violates RFC 3264, Section 6 when handling video call requests. During testing with Yealink and Linphone, Linphone sent a valid video INVITE with SDP:

m=video 44895 RTP/AVP 96 97  
Zoiper responded with a non-compliant 200 OK SDP:

m=video 0 RTP/AVP  
RFC Violation:

RFC 3264 mandates that if a media stream is rejected (port=0), at least one payload type MUST still be listed (e.g., m=video 0 RTP/AVP 96).

Omitting all payload types (as Zoiper does) breaks interoperability with RFC-compliant UAs.

Request:
Update Zoiper to include a payload type in rejected video streams, per RFC standards:

m=video 0 RTP/AVP 96  
Key Points:

Clearly cites the RFC violation.

Provides test evidence (Linphone vs. Zoiper SDP).

Specifies the exact fix required.