with/to

[panicmonger]

Do you accept both of them?

Helen is on the phone with/to Alice.

[Rover_KE]

Yes.

Rover

[bhaisahab]

I'd prefer "to" unless Helen and Alice are together and phoning someone else.

[emsr2d2]

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".

[SoothingDave]

I would say "with."