To customize your Phone Calls:

Go to Channels > Choose the Phone Calls tab

The first thing you need to choose is:
  1. The colour for the sides of your phone call (specially if your exercise is running in a tablet or laptop)
  2. The background colour for your phone calls.
  3. A logo, only if you want to.

The second step is to choose a colour/style four your "accept" or "reject" buttons:
It is also possible to upload your own button designs: