Or more people might use GiffGaff...?
I've set up 1 GG mobile and will be moving another away from o2 when the contract's up, mainly because of the limits on internet usage (i.e. no voip / streaming).
In order for people to abuse using voip services, they'd either need to be paying for the data from their credit or paying £10 a month by means of a goodybag, so I hope that it wouldn't cause GG problems.
Those are just my thoughts though...