My Beagle lost his hearing over the last two years but for a while before that he absolutely hated any metal on metal squeeks. Clothes hangars on rods, the swivel on my arm chair, squeeky doors. They would all make him immediately get up and leave and he wouldn’t come back. I had to eliminate the source of any squeeks. This was mainly a problem after he turned ten and stopped being a problem once he couldn’t hear anymore.

My other dog was a similar age and lost his hearing just ahead of the beagle but squeeks never bothered him so it’s hard to say what it was that bothered the beagle about them. I think given his age when he started reacting negatively he may have already had some hearing loss and the squeeks were at a range that was aggravating or painful for him or he couldn’t perceive them properly and they were confusing and therefore scary.

Dogs can also be very attuned to us so it could also be the dog reacting to how you react to the phone, but since you mentioned the dog is senior I’m inclined to believe it’s something related to her hearing.

I would also suggest you avoid trying to reassure or comfort her when she reacts negatively to the phone. Comforting a scared dog can reinforce the behavior by confirming they are right to be scared. Same thing with fireworks and thunder. Let her leave the room if she wants and take whatever steps you can to eliminate the noise she doesn’t like, but otherwise ignore her reaction.