Phone calls will definitely work. Even if they didn't 'enable' them (which definitely won't be happening), Skype/Whatsapp etc all afford voice calls over data.
I very much doubt many people will make calls considering how noisy the tube is. You can barely have a conversation with someone a lot of the time sitting right next to you. I've been to many cities which have full coverage on the subway/underground and it's very rare to see people on the phone.
Desperately need to get data working though - it's such a pain if you're doing a long long tube journey not being able to read the news, get through emails, know if plans have changed wile you are en route, etc.