A new option to enable the use of Reliable Provisional Response (100rel) in UAS if the UAC supports it (thanks Marcus Froeschl for the suggestion)

[bennylp]

If we receive in incoming call and the INVITE has "Supported: 100rel", PJSUA does not send 180 with "Required: 100rel", so reliable provisional responses are not used. This is because pjsua_call_on_incoming() function does not seem to check whether the received INVITE actually contained a "Supported: 100rel", but it just checks the account configuration's pjsua_a c_config::require_100rel before invoking pjsip_inv_verify_request2() :

   if (pjsua_var.acc[acc_id].cfg.require_100rel)
       options |= PJSIP_INV_REQUIRE_100REL;

So even although the partner supports 100rel and PJSUA supports 100rel (both callee and caller support it) it is not being used in this case unless 100rel is explicitly *mandated* by pjsua_acc_config::require_100rel. However enabling pjsua_acc_config::require_100rel has much stronger implications: Not only does it affect behavior as UAS but also as UAC, and it would make communications with partners who not support 100rel impossible, so it does not seem to be a valid solution to achieve the desired result.