Hi AdHominem,
Thank for your reply. No I'm using my Yealink phones in a bridged mode, so they're in the same subnet without NATing. I also can't say which extensions are affected, as there's (lets say) quite a large group of extensions (about 15 of them) and they're randomly broken, constantly about 7-9 extensions have problem at the same time. but maybe after a half an hour, the extension with problem suddenly registered by itself, but other extensions starting not to work. So, as I say, it's veeery strange issue.
You're right it mostly seems that the problem is with phone registration time-out, so sometimes they're registered, sometimes not. Though the phones always think that they're registered and an account status is always OK and registered, but the server think in another way, by that time and extensions have UNKNOWN status.
When you suggested to decrease phone registering time, do you mean "Server Expires" parameter in an account tab? Now it's set to 3600 (seconds I guess) is 1 hour, should I change it, or there could be any other setting?