Basically using some off the shelf components, the author build a simple box to take the stereo out from an old phone and pump it thru the speakers. What was interesting was he needed to use 2 different USB power supplies due to the noise of just one when it powered both the speaker and the phone.
https://www.instructables.com/id/Reuse-an-Old-Phone-and-Old-Speakers-As-a-STEREO/