What is the ruling on buying a phone in installments then selling it for cash to obtain money, with the intention of paying the installments from another country through an agent?

Purchasing a commodity not with the intention of using it, but rather to sell it and take its price, is called Tawarruq, and it is permissible. As for traveling for someone who has a due debt or a debt that will become due during their absence, it is permissible if they provide the creditor with a pledge for the debt, or bring a guarantor on their behalf. Therefore, there is no impediment to your travel if your friend guarantees your debt and pays the installments on your behalf.