This is a "triple-play" service -- broadband internet plus TV plus (wait for it) phone through the internet cable ... and that's why you need to plug your phone into it.
You don't have to buy all three services, you can buy them in any combination, or just internet. I assumed she needed to plug into the wall jack because she had not purchased the phone through U-Verse. If she has this service then definitely the service guy should have hooked her up. Actually I can't imagine him leaving her without a working service that she had paid for. So that's another clue that made me think she had not bought the phone service - it would not have been his responsibility to connect her land line at that point (although he should have done it anyway).
HBD
Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for... I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca
"Nothing is easier than to persuade people who want to be persuaded and already believe." - another Petrarca gem.
Ignorance is a most formidable opponent rivaled only by arrogance; but when the two join forces, one is all but invincible.