Windows Mobile 6.1 voip

Hello All,

Can anyone here shed some light as to why it seems all of the Windows Mobile 6.1 voip cabs do not work exactly as expected with asterisk? For example on my htc touch pro phone i can call and be called but after 20 seconds the call drops. I have done several debugs in the asterisk-cli and it appears the touch phone simply hangs up after 20 seconds exactly.
Also, the winmo phone always shows unreachable. I have tried every possible config on the winmo config utility and the phone always shows unreachable when doing a sip show peers asterisk-cli.
I have also tried portsip ,fring,aegphone with the exact same results,other than these produce some lesser quailty audio while connected.

Thank You,