Is it permissible to use telecom offers that include a credit increase or free minutes, and is it permissible to benefit from a credit transfer service with a certain percentage deducted as fees?

There is no harm in subscribing to telecommunications companies' offers such as "increased balance for a certain period" or "free minutes." Whatever exceeds the natural balance or the due minutes is a gift from the company. Likewise, there is no objection to transferring credit from one mobile phone to another for a known and agreed-upon amount or percentage.