Никаких 406/488, просто "все заняты". Учитывая ответ сервера укртелекома "500 Internal Server Error", полагаю, их сервер некорректно обрабатывает эту ошибку-- Got SIP response 500 "Server Internal Error" back from 195.5.0.83:5060
Transmitting (no NAT) to 195.5.0.83:5060:
ACK sip:425521@cloud-pbx.ukrtelecom.ua SIP/2.0
Via: SIP/2.0/UDP <asterisk ip>:5060;branch=z9hG4bK386faa45
Max-Forwards: 70
From: "user1" <sip:619F******245@cloud-pbx.ukrtelecom.ua>;tag=as37ce985a
To: <sip:425521@cloud-pbx.ukrtelecom.ua>;tag=fwvenyu19v
Contact: <sip:619F******245@<asterisk ip>:5060>
Call-ID: 16e9ccd0172cc6fc42514757423efdc8@cloud-pbx.ukrtelecom.ua
CSeq: 103 ACK
User-Agent: Asterisk PBX 13.6.0
Content-Length: 0
-- SIP/619F******245-000000ab is circuit-busy
== Everyone is busy/congested at this time (1:0/1/0)
-- Executing [*81425521@internal:3] Congestion("SIP/user1-000000aa", "") in new stack
Really destroying SIP dialog '16e9ccd0172cc6fc42514757423efdc8@cloud-pbx.ukrtelecom.ua' Method: INVITE