VoIP not working properly

[lovingaditya28]

Hi Experts
It has been 4 days and we have issues with goautodial dialler functionality in our network. It sometime work and suddenly stop making calls. Dialler person is saying that this is a firewall issue. I have opened SIP, RTP and 10000-20000 ports range on WAN and OPT both.
Not sure what is going on, sometimes it works and all of sudden stops working.
please find attached WAN rules snap shot. Am i missing something on firewall, if this would have been a issue with Firewall then how come calls works for some time.

kindly shed some light, waiting fro your valuable inputs

thanks