The responses you received sound wrong to me. You certainly need to call the dedicated OB line rather than the normal telesales one, but they should be able to do anything with an OB booking.

If I were you you I'd try to call back again (during UK office hours) to see if they'll do it.

Failing that, if you have enough OB points, you could just rebook it , and then cancel the original booking. Presumably the cancellation process is more straightforward than a change, for them to be able to handle offline.

If none of those works, you could email one of the OB sales execs to see if they can get someone to do it, if you know who your company's dedicated contact is. (PM sent in case you don't have one).