I changed the SIP port in the Grandstream from it's default 5062 to 5060 and now it's using a random high port instead of 5060:
[2016-03-07 19:45:59] NOTICE[1584]: chan_sip.c:28353 handle_request_register: Registration from '<sip:userid@devicename>' failed for '10.0.2.22:19039' - Wrong password