Student or Learner
Do you accept both of them?
Helen is on the phone with/to Alice.
I'd prefer "to" unless Helen and Alice are together and phoning someone else.
I also prefer "to", but this might be a BrE vs AmE difference. I find the same difference when using "talking to/with", "speaking to/with". In BrE, we tend to use "to", whereas my American friends usually use "with".
I would say "with."