Thank you very much for your time Niels.

Finally I found the problem.

The Gateway I have configured in my Innovaphone to route the calls, had activated the RTP Encryption (this parameter is set by default).

It is no matter for my Ascom phones for example, but for the yealink it was a problem. If I don't say to the Yealink phone that exists an SRTP encryption, the innovaphone gateway automatically release the call, because the sent data was not encrypted, it should be a security policy.

My choice was to disable the RTP encryption from the IP800 gateway, however, it could be enabled in the phones (in Yealink settings: Account - Advanced - RTP Encryption (SRTP) = Optional)

I know I am not in a Yealink forum, sorry, but I think it can be helpful for some people because this parameters are default settings for Innovaphone but in the most case of SIP phones (not only Yealink) it is usally disable, and could affect, not only to the music on hold, also to other RTP data.

Hope it can help.

BR

Alejandro