Is it permissible to purchase a phone in installments with the intention of not paying the full amount and taking advantage of a clause in the contract that allows for its termination upon leaving the country?

Purchasing on credit for a specified term is permissible if the debtor intends to repay it. For whoever intends to repay people's wealth, Allah will enable him to repay it, and whoever intends to destroy it, Allah will destroy him. Therefore, it is not permissible to purchase a phone with the intention of not paying its full price, because this clause was put in place for emergency situations where the buyer might be unable to pay, not for evading debts. Your taking others' money with the intention of not returning it is forbidden.