Note to iOS users of app version 2.0 and 2.0.1
Unfortunately a mistake in the app can cause Santa to sometimes stop talking during a received call, when his name is configured as "Father Christmas". Please update the app to the latest version. Apologies for any inconvenience.
Always make sure that you've updated to the latest version of the app.
Make sure your device is not set to Silent Mode or Mute, and make sure the volume is up. If that didn't help, try restarting your device.
Location of the mute button on iPhone and iPad
I've tried the above but I still can't hear him!
If this didn't help, there is an option in the settings screen: "Alternative sound mode". You can try to toggle this to see if it helps: if it was off, try turning it on. If it was already on, try turning it off.