Two things. One, I found the "port=5062" setting in the FreePBX trunk screen. So I'm trying that. And by authority do you mean user and password? Yea, I've checked and double checked.
Now the wiki comes up with this for the HT503:
And I realize I haven't setup an extension yet. But it says to use chan_pjsip and somehow chan_pjsip is disabled.