I had an odd problem a few years ago on a new 200EL install. 911 calls would go thru but not all calls had audio. We had 12 lines in the system and there was no pattern to the problem. Sometimes a trunk would work and sometimes it would not. After working on this with the telco for over a week they found a bad trunk at 911. 911 said we were the only place having trouble. I never could get an answer as to why we were the only one that was dialing in on there bad trunk.