Comcast approves a number of different modems in terms of both brand
name and modelno; Panasonic has a myriad of phones.  The reason I
requested the modelno/s is to determine exactly what you have, not
'sortof' what you have.

Did you get your gateway from a self install kit or some other way?  Do
you have a Comcast XFinity plan or something else?

Does that mean that you are going to get a different modem to enable
voice or does that mean that your current as yet unidentified modem can
do internet telephone?

Also later you said:

Some so-called gateway devices consist (internally) of a modem and a
router (because the router's job is to assign NAT IPs to the devices)
but no ethernet 'switch' which a typical wireless has.

Such gateways typically have only one ethernet port instead of a number
like 4 which most routers which are ethernet and wifi have.

And because you have not specicifically identified either your
modem/gateway nor you Panasonic, no one can comment intelligently on
your original question.

When a question asks for the name and modelno, that means the exact
brand name of the device as well as its model number, which sometimes
requires some careful inspection to determine.

--
Mike Easter