SIP Destroying calls parameters


I configured a SIP remote conections to Asterisk, additionally Asterisk is connected via IAX2 to a VOIP service provider.

If i call too many times from my remote conections, the call seem to broke the first time and took a long time in order to establish the call.

However if i wait some seconds (15 or more) betwen each calls, the new call establish faster and without disruption.

With “sip debug” i have found someting like “scheduling call destruction in 15000 ms” .

Is this situation delaying my calls? How could i setup the schedulling call destruction to fewer seconds?