Is someone who finds a lost phone entitled to claim 10% of its value as a reward?

The person who finds a phone has no right to demand a percentage or anything else merely for finding it, unless the owner voluntarily offers something. The finder is voluntarily offering their services, and if the phone's owner compensates them, that is good. However, if the found item requires expenditure to preserve it, the finder may spend on it and then claim reimbursement for those expenses from its owner.