Like you said beauty is very subjective. But there are 2 ways to look at it.

You can talk to someone on the phone and both of you may feel the other is beautiful. This feeling of beauty comes from Hearing the other person's voice and from processing their thoughts, so the mind gives signal of beauty. This also have to do with you imagination and psychology, because some treats in you made you feel comfortable to the type of talk your are hearing.

And we have the classic way of seeing beauty: The eye!

Following the example above, If you stay in touch via phone for a longer period chances are when you meet your eyes will see more beauty than if you meet without you knowing each other.

So there is your eye, your imagination, your psychology, and many more elements, like your social relationships, culture, etc. etc. That could determine what is beauty for you.