The last 3 that I've booked including 3 calls to get prices and investigate options were to the standard QFF number on the back of my card. Did not bother with the masa line.
When I've booked these I've phoned the premium number and not bothered with the mASA one and the first thing that I ask is whether the person is working on the premium desk. If the premium lines are busy you end up with people on the general desk and you are more likely to struggle with a mASA from that point on. The reps on the premium desk know how to handle these by now and the booking can be made quickly from my experience.