David,
It is hard to say, unless we know your implementation or layout of your IP network.
Are you using SER with any NAT traversal solution such as RTPProxy or Mediaproxy?
Is your UA are behind NAT?
is anything change since you took charge or UA always loosing connections and now you there to fix this existing problem?
Personally i never see UA looses any registration once you configure SER in proper way. Yes, i do have these problems with Asterisk once in a while.
Is there any Firewall in midle of UA and SER?
by the way what is IAD?
Thanks Ali...