SIP Hangup after critical packet

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I have asterisk 1.6.13 running. My ATA are connected via vpn (openvpn) Out going calls from asterisk to the ata works fine Incoming calls from the ata to asterisk cuts off with the error msg Maximum retries exceeded on transmission 

I been googling this error and it was mentioned to use t1min= 500 however its only delaying the problem.

Any ideas on what is the cause of this problem.

Only 2-3 atas are having this problem the rest are fine

zakir