Help identifying supported phone systems for Business Voice

I posted this last night but it was marked as spam.  Not sure why, but I could use some help.

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Hi all,

(My terminology might be off so be gentle)

Short version:

I need help understanding what type of phone systems will work with the 8 phone ports on our Technicolor CGA4131COM router/modem.

What we want is to be able to have various offices with desk phones and be able to make/take calls, with the main office having the base station, preferably over the internal network.

Long Version:

My wife manages a small outpatient therapy business.  They don't have an IT person so I help as needed.  One of their offices has Comcast Business internet and due to poor voice quality recently switched from Vonage service (I believe) connected via a Yealink W52P base to a Comcast Business Voice line.  They currently only have 1 line (phone number) but may go to 2 total.

We connected an analog phone using the RJ10 plug to port 1 on the router and it seemed to unexpectedly work.  I thought it needed an IP phone of some sort.  

I have searched like mad to find any list of recommended phones on the Comcast site, for the supported protocols, to see if IP phones are supported or for component diagrams of various configurations but I've come up empty.  

The building they are in has a really old Avaya PBX (it has a 3 1/2" floppy for voice mail greeting recordings I presume) and the handsets that work with it are no longer available (according to the person we talked to).  Using/configuring a PBX is way out of my comfort zone.

So in summary, I need some help connecting the dots on what devices work in terms of the phone systems that can connect to the Technicolor CGA4131COM phone ports.

Many thanks!