Tried... I set to 60s, when registering the Optipoint sent 72s then and the server overwrote to 120s. Anyway the phone never tries to register again, neither after 60, 72, not 120 seconds.
But... could you take a look at http://more.voip-sip.info/index.php/SIP_V6.0_optiPoint_compatibility ?
It...