INVALIDNMBR while calling via IAX2 trunk

I have two pbx (recent FreePBX distros) which are connected with IAX2 trunk. Call routes work quite good but sometimes I see in asterisk log: “Dial failed due to trunk reporting Address Incomplete - giving up”, INVALIDNMBR, “Dial failed for some reason with DIALSTATUS = CHANUNAVAIL and HANGUPCAUSE = 1”. I couldn’t find what is the nature of the problem, because most of the connections work as expected.