Are the additional amounts imposed for late payment of phone bills or university fees considered usury, and is it forbidden to pay them?

Yes, this increase is undoubtedly usury (riba). The longer the repayment period, the greater the amount becomes. Therefore, one must refrain from this transaction. However, if you are compelled into it, then pay the original amount without the increase. But if you are forced to pay the increase, then pay it, accompanied by repentance and a commitment not to return to such a transaction.