Is contributing to the sale and purchase of prepaid calling cards, at a fixed price and a fixed weekly return, with the possibility of withdrawing capital at any time, halal or haram?

If the periodic return is fixed at a lump sum amount, unaffected by profit or loss, with capital guaranteed, then it is impermissible to invest in this company. This is because partnership necessitates that partners are exposed to both profit and loss, and pre-determining the profit invalidates the contract.