Is working in the marketing of telecom companies' offers, which include selling devices in installments, and buying devices from customers at market price while they continue to pay their installments, with the possibility of the customer obtaining larger sums by purchasing multiple offers and paying their installments, considered permissible or does it fall under usury (riba)?

Selling the phone along with calls and internet is permissible. However, the company buying the phone back from the customer for an immediate cash price is forbidden because it is a Bay' al-'Inah (a form of 'ayn) which is prohibited. Therefore, if the work is limited to marketing the sale of the phone, calls, and internet on installment, it is permissible. If the work includes marketing the purchase of the phone from the customer, it is impermissible.