With the setup you have at the moment (Vonage and BT Landline) it is not possible to get both providers working with your GO basestation as they both present as an analogue line.
The GO box can handle one analogue line and up to 6 SIP based VOIP providers. Vonage do not supply SIP credentials to residential users so you cannot configure the GO unit to setup Vonage as one of these 6.
You could upgrade to the Vonage Business service (at greater cost) and then you'd get the SIP details.
The best suggestion would be to abandon Vonage if possible and setup a proper VOIP service with someone else. As another poster mentioned, Sipgate is easy to try.
Otherwise, as you're investigating, you'll need to port your BT number to a VOIP provider (not Vonage!!) so you can have Vonage plugged in the PSTN port and your BT number via one of the 6 available VOIP configurations.
You mentioned that you are aware that your BT line will generally cease when you port the number so it is worth double checking how your broadband is supplied. Do you have an AAISP organised line in addition to your BT line?
AAISP Home::1 Terabyte | IPv4 BQM
| IPv6 BQM
| AAISP VOIP | ER-Lite Router | Unifi AC-Lite Wifi AP
Edited by ferretuk (Tue 09-Jan-18 09:12:14)