Connecting the Cisco 7281 IP Phone to Xfinity gateway router? Setup steps for phone?

I have an Xfinity gateway router.  I have Internet, TV, and telephone service.

I already have a line connected from the RJ-11 POTS port on the gateway to the RJ-11 jack on the wall.  That all works.

Now I want to also plug in a Cisco 7281 IP phone into the gateway router without disabling the POTS ports.  That means I want this IP phone to be an extension of the same number, just like the other analog phones in the house.

Questions:

1. Is my telephone number linked to the public IP address of my gateway, or to the MAC address of my gateway?

2. Does Comcast need the MAC address of my IP phone?

3.Is there anything that needs to be configured within the firewall of my gateway to ensure that the IP phone service will transfer to one of the Ethernet ports on the back of the gateway?

I know that it is possible to do this because a Comcast customer service agent said that he is taking calls from home, and his IP phone is plugged directly into his gateway, and his ports from the ATA are plugged into the wall so that all of the analog phones in the house will have service.

4. Will I be able to add additional IP phones in the future that all work as an extension of the same number?