[Serusers] Some calls do not go to voicemail

We are experiencing a problem for which we can use some help. When a subscriber receives a call from either the PSTN or another IP phone we set the inv_timeout to a value which represents about four rings. For most subscribers an unanswered call will timeout after the specified number of rings then, via a failure route, their call will be re-route to our Asterisk server for voicemail. Some subscribers, no set pattern of users, have their calls timeout and the caller experience silence for about 45 seconds. During this period the caller usually hangs up. I cannot seem to figure out why. We set modparam("tm", "fr_inv_timer_avp", "inv_timeout") and then avp_write("20", "inv_timeout"); before relaying an incoming call. Has anyone else experienced any similar problems? Thanks,Steve Senior Network Engineer, Information Systems and Computing Networking and Telecommunications , Suite 221A /6228 University of Pennsylvania Voice:215-573-8396 FAX:215-898-9348