Hmmm...
Well, it is my understanding that your (and all of our) VOIP packets will NEVER be resent EVER. The packets sent are ALWAYS UDP over RTP and cannot (that I know of) be changed to TCP. This would take too much time to do and would add to delay problems. So...

a) IPSEC has to re-negotiate a security key every so often which interupts data flow. What is the interval?
b) What type of routers are you using?
c) how many VPN's are you using?
d) Is this a teleworker app or pbx networking?
e) G711 or G729?
f) how many concurrent calls?

Dave M