Try 0208 745 4151 or 0208 745 4152.

However, in my experience, no one ever picks up or it goes to answerphone saying that there is no one available.

Did you ask reservations to link your bookings together?

I suggest you try and rever to Reservations and ask them to help you. Maybe they can put a note in both your PNRs (bookings) for you.

I hope you have a good flight on board SQ 0319