What is the ruling on a loan transaction for purchasing goods from a relief organization that buys merchandise (mobile phone cards) and sells them to me at a fixed profit, with the amount repaid in installments and guaranteed by employees of a Riba-based bank?

Murabaha with deferred payment is permissible if the price and profit are known, and if there is no increase in the installment amount due to late payment; otherwise, it is usury (riba). The fact that the two guarantors work in an interest-based bank does not affect the permissibility, as long as you are not obligated to open an account with it.