Since you say you have a phone line, somewhat recently (past couple of decades) installed, you should have a Telephone Network Interface Device box, which is typically located on the exterior of the house and labelled as such. The "network" referred to in this case is not "the internet" but rather the "telephone landline network" which can be used as one way to get to "the internet." Any wiring inside is very unlikely to be the telephone company's for something like 35 years now. While you may have a box inside that used to belong to the phone company, and your house may once have had 3 phone lines, this is very unlikely to be the active service in place now. It may or may not be connected to the active service.
The Telephone Network Interface Device is also called the "demarc" (demarcation) point. It has a portion intended to be opened by the customer (you) with normal tools, and a portion intended to be opened only by the company with special tools. Inside the customer part there are telephone jacks (which may have lines to the inside of the house plugged into them) - if you plug a phone into those jacks, dial tone indicates that you have service. If you don't have dial tone inside, you have a problem with "customer owned wire" and fixing it is on you. If you don't have dial tone at the interface, and you are paying for a landline, that's on the phone company. If you never attached a phone to the line you had, it may be that the lines into the house were never connected to the interface box when it was turned on.
In most areas it is still unusual for there to be access to more than one landline telephone provider, despite rule changes that were supposed to make this possible - so I am a bit dubious that you had an actual landline from AT&T and yet Verizon is offering you DSL (which uses an actual landline) at the same house. You may have had some sort of "internet telephone service with a landline interface on it" but it's really hard to say if you never actually used it, which obviates asking such pesky questions as "did it work when power was out" (a real landline will, unless the phone line is out.)