In this case we need few more information to assist you on this.

Could you please help us with the model number of the phone that you are using.

Are you connecting the phone directly to the modem phone port or to the phone Jack separately?

Also have you activated the modem phone port by contacting your service provider?

Thank you for responding.  I will try to clarify the issue as follows:

The Arris modem has two phone ports. I can attach a phone to either one of them and the phone will work properly. I am using an Alcatel captioning phone. I suspect, but have not attempted to attach a second phone to the second phone port and both phones would work, but that is not what I am trying to do. I would like to connect the phone lines that are running through the apartment to the modem so that a phone plugged in to a remote jack can act as the second phone.  I think that those other lines are already connected to another source though, which is not activated, and the circuit is thus unable to work with the modem. In fact, just connecting it to the modem prevents any phone to function. I think this is just a lost cause and I should have been able to realize it initially.

Basically, T25 is a cable modem with DOCSIS 3.1 with two telephony ports for digital voice service. Activate both the ports with help of MTA MAC ID which is available at the bottom of the modem.

In regard to the connection of the various phonelines running through the apartment would suggest in approaching your ISP for further clarification as they support on more information about voice lines.

If any issues on no internet/slow speed /intermittent connection, please free to post on the same.