iPhone requires an Audio Source selection every time I answer a call

My friend's unlocked iPhone (he got it overseas) no longer just let's him answer a call when the phone rings. He now has to answer it, then touch the Audio Source button, and then there are three options--Motorola H371, iPhone and Speakerphone. Only after hitting the iPhone button can he have a conversation. It wasn't doing this yesterday.

How can this be fixed so the audio source is auto-selected, and answering calls just works.
 
indifferent I’m stymied
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