You said you have high speed cable, so an external connection to phone service needs to be disconnected and maybe tagged to say leave unconnected.
Sometimes a discontinued phone may have emergency services left, or is not disconnected when you get your new service, so it is still active.

I always called to cancel the old service when my number was ported, and working fine.
I prefer to leave the wired bell technology to history and stick with a wireless base phone off the ATA (Analog Telephone Adapter.)


Two devices should work fine. If you had a bunch of wired phones that exceeded the REN (Ring equiviliency number) then some may not ring properly.
Five old school phones is usually supported.