As an aside, I had service with a local VOIP provider and ported away. When the port showed up at the new provider, the phone rang at both providers. I modified my adapter to the new service and never looked back. Well, almost never looked back...
Weeks later (or was it months later) I plugged in an old adapter I had and it turns out it was still configured with the old VOIP company. It still had a dial tone and when I made an outbound call to my cell phone, the caller ID showed it was still an active account. I have not checked lately but that old VOIP account may STILL be useable. Who knows what happens if there is actually time allocated to calls. Maybe they would try to bill me.
Yet moving further aside, I had local POTS service here, but cancelled it when I got VOIP. I disconnected most of the house but there was still a home run to a phone downstairs in an area I rarely used. I plugged a phone into it one day, got a dialtone and dialed upstairs. When I looked at the caller ID it showed someone elses number and name. Turns out the phone company reused the wires but never disconnected them from my house, thus I could have listened into the neighbors calls If I wanted. It has been 4 years, but I'll bet that line is still active! People wonder why there is noise on their POTS line when it may be miles of extra wire paralleled to their line.