I was thinking. The phone you get receives one of three random calls. Alphys can order a pizza, Sans can Prank call you, or you can get the wrong number number song call that was intended for someone who's name begins with G.

It's been commonly been theorized that the other two Gaster is talking too in entry 17, is Sans and Alphys.

It would make sense, that Sans and Alphys Have the phone number, and probably don't know who it actually belongs to, since Gaster has been, well, for the most part, removed from the game. and there really is no way for them to have your number at the times they can call you.

The only thing i can't really put a finger on is how Toriel came to possess Gaster's phone (if, you know, it is Gaster's phone) perhaps she took it before she left New Home?