Is it obligatory to return the mobile phone and camera received from the defaulting contractor who abandoned the site, and to whom should they be returned in the absence of his representatives?

The items you have are a trust (amanah), and it is not permissible to dispose of them except with the owner's permission. If he permits you to take them, then you may do so. Otherwise, they must be returned to him. It is not permissible to give them away as charity except in a case where you are unable to deliver them to their owner and have despaired of identifying him. In such a case, you may give them away as charity on his behalf, just as 'Abdullah ibn Mas'ud did when he gave away the price of a slave girl as charity because he could not find her owner, saying: "O Allah, on behalf of the owner of the slave girl."