When receiving calls if there is no sound, change in Ford Sync: Phone setting -> Ringtone -> Use Phone Ringtone option must be chosen.
If that does not work try the following:
(see Recommended Answer): https://support.google.com/androidauto/thread/4389113/no-sound-for-incoming-phone-calls-pixel-2-on-ford-sync-3?hl=en
Install the Sync Update 3.4 Build 21194.