What is the ruling on telecommunications companies' offers that grant additional credit for a limited period, after which it becomes zero?

There is no legal impediment (shari) to the transaction mentioned. The communication service provided by companies falls under the category of ijara (selling a benefit), and there is no harm in setting a specific duration for it. When the duration ends, the full rent (or fee) becomes due, regardless of whether the benefit was fully utilized or not. Jurists have explicitly stated this, and the Permanent Committee for Issuing Fatwas has issued a fatwa to this effect.