What is the ruling on someone who signed a contract with a telecommunications company to obtain a phone in installments, then closed his bank account to evade payment in a non-Muslim country, and how should he repent?

The impermissibility of this act is clear; it is a form of betrayal and consuming people's wealth unlawfully. The fact that a person is in a non-Muslim country does not justify it. It is incumbent upon whoever did this to repent to Allah and to pay the remaining installments of the phone to the company they contracted with, by any means possible, even if they have left the country.