Hi Stuntshell,
There isn't really any well-defined support in Asterisk for an outbound sip proxy, even though most hard and softphones have it.
You can generally get something to work through a combination of complex registration strings and a few vendor "type" specific asterisk patches.
In general, it tends to be the "lockdown" VSPs who use outbound proxying because they want you to use their specific hard or softphone(s). Unless there is some absolute imperitive to use such a carrier then why bother? There are usually excellent alternatives that are more asterisk "friendly".
Kind Regards
Selintra