Hello, thx for your reply.
SIP trunks are configured in strict correspondence with the
https://downloads.avaya.com/css/P8/documents/101022915 document. I've additionally checked the properties of Element Manager -> Avaya Aura Core -> Session Manager and the checkbox Trusted is set. The connection policy field in SMGR -> Routing -> SIP Entity -> Conferencing is set to trusted also. Any other ideas? Thank You!