bank's computer doesn't recognise hash-key during two-factor identification

When the bank's robot calls to collect your one-time code, it asks you first to tap the hash-key. The bank's machine can't hear this key.

The bank says the problem is with the phone-network and says: "You should tell [the network] that you are failing your banking security process due to it not recognising the tone when you use the keypad when you get a security call. It is due to [the network's] poor-quality compression of reverse DTMF files." The network say the problem isn't to do with their system but with settings on the iPhone.

Who's right, please, and what can we do? Thanks and best wishes.