Very long setup time on outgoing sip calls


I have the following problem: SIP calls made between asterisk registered users (for example, calling extension 1667, below) are setup almost instantly.

exten => 1667,1,Answer
exten => 1667,2,Dial(SIP/pblackman)
exten => 1667,3,Hangup

But if the peer is outside Asterisk´s domain (see example below), setup time takes at least 6 seconds, and during this time there is no ringtone, unless I use the Ringing command on the dialplan. If I call userx@ directly from a softphone (not through asterisk), call setup is very fast.

exten => 1668,1,Ringing
exten => 1668,2,Dial(SIP/userx@
exten => 1668,3,Hangup

Am I missing anything? Any extra parameters or configuration issues?

Thanks in advance.