There is info in the call set-up packets that tell a receiving device whether or not to go to speaker. Once the call is set up, though, it shouldn't matter how the audio is being rendered. It might help to know if it happens at both ends of a call, if it's only on incoming calls, if it's between similar or different devices or if it happens only on internal calls or if outside calls act this way.

Sometimes the thoughts in my head get so bored, they go for a stroll through my mouth. This is rarely a good thing.