When b2b creates topology hiding scenario, it setups too small call setup time:
/* if it will not be confirmed -> delete */
tuple->lifetime = 60 + get_ticks();
When alice calls bob and bob don't answer for 60 seconds, b2b send CANCEL to both sides.
I think we should give possibility to configure this parameter, either in AVP or b2b_init_request flags like b2b_init_request("top hiding/t300").
I'd can to write the patch after discussion about what method to use.