instead of exten=> s,n, Dial(SIP/somenumber) i tried
exten => s,n, AGI(mytest.agi|somenumber)
with the following ultra-simple agi-script that does nothing but dial out (and two console messages):
-------------------------------
#!/usr/bin/perl
use Asterisk::AGI;
$AGI = new Asterisk::AGI;
my $nummer=$ARGV[0];
print STDERR "GAGA $nummer GOGO\n" ;
my $res=$AGI->exec('Dial', $nummer );
print STDERR "GUGU $res GOGO\n" ;
exit;
--------------------------------
however, it's not the same as the dial command:
when the callee takes the phone and hangs up,
it's ringing again at the callee???
note: the next line in the dialplan is: 
exten => s,n,Hangup

any clue?