Ok, the 64 seconds is a result of the retransmission timers set at the SIP stack level which can also be modified by the application when passing a SIP_STACK_CFG structure into the IP_VIRTBOARD pointer. Refer to chapter 10 sip stack cfg section in the GC IP TUG here:
www.dialogic.com/.../globalcall_for_ip_hmp_v12.pdf
Jeff