Asterisk dropping calls


We have been using Asterisk for some time and just recently strange problem emerged.
Quite often calls are dropped after random amount of time (ranging from less than minute to a couple of minutes) . The only suspicious message that appears in logs is something like:

Nov  9 11:41:39 WARNING[11999]: Maximum retries exceeded on call for seqno 1 (Non-critical Response)

Clients are connected to Asterisk via IAX (idefisk softphone), not SIP. Calls are made to pstn numbers via broadvoice. Is this problem between iax clients and Asterisk or between Asterisk and broadvoice ?

Any idea how to fix this ?