Is it permissible to incentivize soldiers to search prisoners by giving them a portion of the confiscated mobile phones, knowing that a lack of incentive leads to their lack of enthusiasm and perhaps their concealment of confiscated items?

It is not permissible to incentivize soldiers with confiscated mobile phones, because the officer does not own them, nor is he authorized to dispose of them. A gift is an act of ownership transfer, and transferring ownership of something not owned by the giver is impossible.