This solution has been proven to work on all 6.x.x Firmware releases for NewEdge Networks SDSL routers, This addresses the UDP Session Timeout Value, We have found that 20 min appears to be the standard timeout value for SIP connections which will cause call drops after 20 min.
The solution to this is to input the following via CLI then save all and reboot.
system ppp udp_period 0
system ppp udp_time 0
system ppp udp_context_id 15
system sip setalgport 999
If you do not have administrative rights to the router another solution is to make the transport sessions of the phone use TCP instead of UDP. The following examples are configuration changes to the Polycom Soundpoint IP Phones SIP Config file: