asterisk users - Dec 2004 - extension number when calling to registered gateway

Hi, Im new to asterisk and I have a problem with extensions. If I use this
line in extensions.conf:

        exten => _011.,1,Dial(SIP/{EXTEN}@gateway_ip_address,60)

the gateway receives this:

        INVITE sip:extension_number@gateway_ip_address;user=phone SIP/2.0

but if I register the gateway in sip.conf and I try to replace the ip addres
with the alias:

        exten => _011.,1,Dial(SIP/{EXTEN}@gateway_alias,60)

in this case the gateway receives:

        INVITE sip:gateway_alias@gateway_ip_address;user=phone SIP/2.0

Is there another way to configure the extension so that asterisk includes
the extension number when calling to a registered gateway?

thanks
roge