Mom moved to the Isle at Watercrest and they provide the cable but now I need to add a landline. I know that I must have to add a modem which I want to purchase myself. Anyone familiar with this? What do I need to buy? No internet will be used. Thanks.

Not a cable phone fan, and think you have to use their VoIP gateway, and they likely want one built into their modem. Actually would recommend you get the lowest tier internet and get VoIP Service from a separate provider like voyant for maximum flexibility. My mother has a similar situation at Katy watercress and sister set up phone with Xfinity. Now stuck with substandard cable.phone service and when kids or guest visit no internet and things like her internet connected picture frame and other IP devices are near useless.
If you really don't want internet get a cell phone or something like an att homebase device.