I just transfered one cell phone number to another, they both still work. Why?

Factual Questions
1

I have Verizon and I just switched from one phone to another. Not realizing they were both on, a call came in and a text and they went to both phones. Do both phones have accessibility to my phone number?

Whats the deal? how long should I expect both the old and new phones to work for?

2

I did the same thing! (Lost one, replaced it, found it. It still works.) I am superstitious about having them both on at the same time, though. I haven’t checked for a while, but they both worked six months after I got the second.

Tris