I too had issues connecting this unit to an Allworx 48x running 7.6. Polycom phone software 3.1.4 Boot rom 4.2
After many tries I hit on the secret.
access the admin options by the menu key on phone.
Note: default admin password for the phone is 456.
Choose reset phone to default values.
After phone reboots, access the menu again to find it's IP address or pay attention while the phone starts as it will display it.
open a web browser and enter the Phones IP address.
It will ask for a username and password.
Default Username: Polycom (Case sensitive)
Default Password: 456
Choose the GENERAL tab from the phone's home page.
Goto Microbrowser setting.
Fill in the Main Browser Home entry with your Allworx internal IP web portal.
http://xxx.xxx.xxx.xxx:8080 (xxx.xxx.xxx.xxx being your Allworx server's internal IP address)
Click the submit button. The phone will reboot.
After phone has restarted, open the webserver and goto the SIP tab.
In the outbound Proxy section, enter in your Allworx server internal IP address in the address field and port 5060 in the Port field.
Pick UDPonly in the drop down in th Transport field.
In the Server1 section, again enter the Allworx server IP address and the port 5060. Leave the Transport at default of DNSnaptr.
Click submit and wait for the phone to reboot.
Open the phone webserver and navigate to LINE tab.
This next part will have required you to setup a generic handset in the Allworx server and assigned a user.
In the Display field enter the assigned extension.
In the Address field enter the registration in the form of
xxxx@xxx.xxx.xxx.xxx In example, my entry was 5110@192.168.000.001
In the Authentication User ID enter the registerd ID. In my case it was 5110
Enter the Authentication password in it's field. You did note it when you created it for the user in the Allworx server?
Enter the extension in the label field.
Click submit.
Once the phone reboots it should work. It did for me. Your Mileage may vary.
Good luck.