Serial number is available from 4 places:
On the cardboard box,
At the bottom of the plastic housing around the M1 circuit board,
On the audio amplifier heat sink on the M1 printed circuit board, and
In installer programming, Global 07, menu 43.
The serial number is an 8 digit hexidecimal code.
I used my M1 with Vonage with no problems. I had the M1 setup to transmit central station format Contact ID which is a touchtone format.
The only problem I had with Vonage is the Vonage VOIP modem would lock up and tie up the ethernet which had nothing to do with the M1. Power cycle the Vonage and everything would work for a while. Hopefully the new Vonage systems work better. The one I had was 1 1/2 years old. If I used Skype on my PC the Vonage would lock up immediately. I finally gave up on Vonage due to lack of customer support and went back to Bell South.