Which bank? I've found them mostly understanding when I've asked people if I can call back.
Although they still try to ask for your DOB first, which means you can't have a blanket "If someone calls from your bank and asks for your DOB, they are definitely fraudsters" rule.
Note that fraudsters are doing things like faking caller ID and holding the line open, playing dialling tone on landline calls so that you hang up, think you are dialling again, and actually are still on the original call. The fix there is to use your mobile to call them back, or call someone else first. You can also feed them wrong information and see how they react, but I'd still do the call back thing.
Calling someone else first is a false sense of security: if scammers are faking a dial tone, they will starthaving software to connect the call too, and then resume the scam when you call the bank number.
Although they still try to ask for your DOB first, which means you can't have a blanket "If someone calls from your bank and asks for your DOB, they are definitely fraudsters" rule.
Note that fraudsters are doing things like faking caller ID and holding the line open, playing dialling tone on landline calls so that you hang up, think you are dialling again, and actually are still on the original call. The fix there is to use your mobile to call them back, or call someone else first. You can also feed them wrong information and see how they react, but I'd still do the call back thing.