T23P unknown uri scheme on 8x8 activation

[JJLatYL]

I got a bunch of T23P phones from 8x8 and when I hit the "activate" button, it tries calling 389. It shows a moving heartbeat for about 30 seconds with dead silence and then gives me a busy tone and displays "unknown uri scheme".

I got 3 Polycom IP550 phones at the same time and they all activate perfectly. On the Polycoms, when I hit the activate button, they call 389 and the activation service answers immediately and asks for the activation code.

On my router, all ports are open for particular internal IP addresses belonging to VoIP phones and in addition all ports are open to IP address blocks for 8x8 servers. I've also disabled all SIP ALG functionality. I have a Cisco cable modem from Cox for my 100mb business internet service with port 1 in bridge mode to my router. If I connect a Yealink to a non-bridged port on the cable modem, it does the same thing.

I factory reset and have tried 2 different autoprovision URLs from 8x8 for Yealink devices. One of them is meant for some kind of Yealink DECT device allowed the activation service to answer and ask for the code, but the activation didn't seem to actually take. If I use the URL recommended for this phone, the phone activation fails the same as before.

I had the same problem when trying to activate from my home network which is a residential Cox service. A Tier 2 tech used the same model with a slightly different hardware version and was able to activate on their network.

It seems to me that if this is a bad SIP ALG implementation somewhere or blocked ports, it seems like the Polycom should have similar problems, and the activation service should not have picked up when using the wrong autoprovision URL. Are Polycoms more capable than Yealink at working around bad SIP ALG or blocked ports? Has 8x8 done a better job setting up their SIP Proxy for Polycoms?

What else can I check?