Cox cannot...by law...force you to rent a telephone modem. If you want telephone service from Cox, Cox must freely give you the modem. That it until you cancel service...then they'll want it back or happily charge you $300.
If you have an outdated telephone modem...like I did...exchange it at your Cox Store. However, it sounds like your local Cox Store doesn't have any more free telephone modems, so they have to give you something for telephone service.
Telephone modems are free. Internet modems are not free. You can rent an all-in-one device from Cox for bundled service, that's a rental fee.
A Panoramic all-in-one as an telephone-only device is kinda an overkill, but, perhaps, it could be just temporary until they get more units in stock.
...if your current Cox telephone modem works, just hang on to it until more are in stock.