Praise be to Allah the Lord of the Worlds.
Stock exchange regulates the issuance and circulation of shares, bonds and securities. It is well known that bonds are usurious loans whereas shares are units of ownership in joint-stock companies.
The function of the "call center" is to execute clients` orders regarding sales and purchases, so it could involve executing lawful as well as unlawful transactions.
Islam has forbidden whatever directly helps in committing sin since Allah the Almighty says, "Help ye one another in righteousness and piety, but help ye not one another in sin and rancour: fear Allah. for Allah is strict in punishment."{Al-Ma`idah, 2}. In fact, the execution of unlawful transactions is unlawful because it directly helps in committing sin.
Accordingly, if the job mentioned in the above question involves executing unlawful transactions, then it is unlawful, but if it doesn`t, then it isn`t. And Allah knows best.