How to receive a call from mobile to SIP using SIPml5 and a browser?

Question

WebRTC

There is a service (SIPml5) that provides the ability to make calls using a SIP account.
In principle, outgoing calls to a mobile phone work. That is, they work according to the scheme:
Chrome / Firefox-> SIPml5-> WebRTC-> 3rd WS (webrtc2sip)-> Your provider-> cell number for the call.
The call is accepted and you can talk.
The difficulty is that the call in the opposite direction does not work.
That is, we call from a cell phone to the number to which our SIP account is assigned, we see an incoming call in the browser, when we try to answer, the connection breaks, and the error I nvalidSessionDescriptionError: Invalid description, no ice-ufrag attribute
is displayed in the browser console tick on the use of technologyWebRTCBreaker - the call is accepted, but there is no sound. From this we can conclude that the potential reason is in the audio codecs and when they are turned off (WebRTCBreaker is designed for this) - the call will be accepted, but without the interlocutor being audible.
Technical support provider OnlinePBX (whose account is used to make / receive) calls - can not yet give a clear answer.
There is no access to intermediate servers. That is, it is almost impossible to understand what went wrong on the server side of the provider or in the service that represents webrtc2sip through which the call also passes.