I believe this is because you're usually expected to setup an SRV record for your domain. The one for my domain looks like this
_sip._udp.simcop2387.info SRV 1 0 5060 simcop2387.info
you can set a nonstandard port there, and then all the asterisk servers and everything else that is setup to lookup SRV records for unknown domains will pick up the nonstandard port without being configured and get you much better compatability, if you can't set the SRV record then i'd suggest changing providers for your nameserver (there is a free one out there
FreeDNS - Free DNS - Dynamic DNS - Static DNS subdomain and domain hosting that lets you do it)