All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
It is not clear to us how the mobile phone credit is used as a price for buying these things. However, generally speaking, the price could be paid in the form of cash, a tradeable item, or a benefit (usufruct).
Al-Hajjaawi underlined that the jurists' definition of a sale is as follows, "It is the exchange of cash in return for cash, even if it is on credit (deferred), or a lawful benefit (usufruct) in return for their like on a permanent basis with no usury or loan involved..." [Al-Iqnaa‘]
Hence, the price could be an object or a benefit (usufruct), and phone credit is considered a benefit. Therefore, there is no harm in using phone credit as a price for buying services or commodities or the like.
Allah knows best.