If all you hear are strange, muffled noises, chances are it’s a pocket call.

pocket call

/ ˈpɒkᵻt ˌkɔl / noun

a call that you make by pressing your phone by accident:
It must be a pocket call – I can’t understand a thing.

The verb is pocket-dial or pocket-call:
It can be embarrassing if you pocket-dial your boss from the bar.

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