Сравнительно недавно появилась следующая проблема: после 30-40 секунд разговора пропадает связь (при этом softphone считает, что линк есть); кроме того, перестал работать трансфер звонков. Проявляется это почему-то лишь с частью телефонов - остальные работают стабильно.

Для всех softphones стоит nat=yes, canreinvite=no. Проблема точно возникает с Linksys SPA921 и Goldstream 2000, локализовать более четко пока не удалось (еще есть несколько SPA901 и много адаптеров SIP-в-телефон).

В логах только вот это:
[Mar 3 19:28:41] WARNING[2890] chan_sip.c: Maximum retries exceeded on transmission 69e6daea-bd1dde8b@192.168.1.190 for seqno 102 (Critical Response)
[Mar 3 19:28:41] WARNING[2890] chan_sip.c: Hanging up call 69e6daea-bd1dde8b@192.168.1.190 - no reply to our critical packet.
[Mar 3 19:47:43] WARNING[2890] chan_sip.c: Maximum retries exceeded on transmission 3790f9900bf17139@192.168.1.213 for seqno 2795 (Critical Response)
[Mar 3 19:55:28] WARNING[2890] chan_sip.c: Maximum retries exceeded on transmission 7760e5a895103868@192.168.230.2 for seqno 48595 (Critical Response)
[Mar 3 19:56:33] WARNING[2890] chan_sip.c: Maximum retries exceeded on transmission 23c9105021cb0c20@192.168.230.2 for seqno 17181 (Critical Response)
[Mar 3 19:56:47] WARNING[2890] chan_sip.c: Maximum retries exceeded on transmission 588f5a6c8b68a874@192.168.230.2 for seqno 17952 (Critical Response)
[Mar 3 19:56:47] WARNING[2890] chan_sip.c: Hanging up call 588f5a6c8b68a874@192.168.230.2 - no reply to our critical packet.
[Mar 3 19:58:24] WARNING[2890] chan_sip.c: Maximum retries exceeded on transmission aff77e6cc2a7a4cb@192.168.230.2 for seqno 7323 (Critical Response)
[Mar 3 19:58:39] WARNING[2890] chan_sip.c: Maximum retries exceeded on transmission 1d131e4c1f999ff7@192.168.1.214 for seqno 46249 (Critical Response)
[Mar 3 19:58:39] WARNING[2890] chan_sip.c: Hanging up call 1d131e4c1f999ff7@192.168.1.214 - no reply to our critical packet.
[Mar 3 19:59:01] WARNING[2890] chan_sip.c: Maximum retries exceeded on transmission 2bb50c17-32e9d9d8@192.168.1.190 for seqno 102 (Critical Response)
[Mar 3 19:59:01] WARNING[2890] chan_sip.c: Hanging up call 2bb50c17-32e9d9d8@192.168.1.190 - no reply to our critical packet.
[Mar 3 19:59:42] WARNING[2890] chan_sip.c: Maximum retries exceeded on transmission 98b164e8536a844a@192.168.1.213 for seqno 29033 (Critical Response)
[Mar 3 19:59:42] WARNING[2890] chan_sip.c: Hanging up call 98b164e8536a844a@192.168.1.213 - no reply to our critical packet.
[Mar 3 20:09:13] WARNING[2890] chan_sip.c: Maximum retries exceeded on transmission f5742805-dec5b2b7@192.168.1.190 for seqno 102 (Critical Response)
[Mar 3 20:09:13] WARNING[2890] chan_sip.c: Hanging up call f5742805-dec5b2b7@192.168.1.190 - no reply to our critical packet.
[Mar 3 20:30:25] WARNING[2890] chan_sip.c: Maximum retries exceeded on transmission 44aa3db0f1bc11a0@192.168.230.2 for seqno 36810 (Critical Response)