Hello,
You might want to increase UDP session idle-timeout from default 60 sec to 2-3 mins or more.
http://kb.juniper.net/InfoCenter/index?page=content&id=KB21344
Unless You know the UDP RTP port range that Your phones use, You'd need doing that for all UDP high ports except DNS (src.port 53, 1024-65535 && dst port 53) and NTP (src.port 123, 1024-65535 && dst port 123)
HTH
Thx
Alex