Is your DH at home or in hospital? If it's home, then he can call and give you permission over his account - my dad did this when I took over his contract. Normally they put a 'password' on the account, once you've got that, you're authorised to deal with it.
If he's in hospital, call back, ask to speak to a supervisor and explain your DH is in hospital and you can't get him on the phone, hopefully they'll be able to help.
It does sound like an error. You don't suddenly get a £600 bill.