Are you defended against a MITM attack? Certainly you are not well-protected. First, you have no assurance that the phone number you dialed was answered by a bank employee. The phone call could have been intercepted by an attacker, impersonating the bank. GSM encryption can also be disabled by an attacker, especially so on an iPhone which violates the GSM standard by disabling the "Warning! Unsecure call!" prompt. So it's possible your call could go out in cleartext to an eavesdropper.
But you asked a more important question: "Is it safe?" That's a risk assessment you'll have to decide for yourself. Do you know if there is an attacker interested in you, personally? Do you have vast personal wealth that would tempt a criminal? The level of sophistication required to perform a MITM intercept would have the attacker looking for millionaires to steal from in order to maximize profits while minimizing risks. If your bank account has only a few thousand dollars in it, you are probably not a valuable enough target to worry much about it.
Finally, to protect yourself and your money, you should look at using a bank with two-factor authentication, where you have a pocket token with a rotating number. The bank would refuse to deal with you unless you held the token, and you would refuse to deal with them if they couldn't authenticate themselves to you.