I've not had any problems at all (Vodafone - on an iPhone of all things!). It's a good question though, because Spa is in a bit of a bowl with a lot of hills nearby.
Never had any trouble with Vodafone (think it is SFR Belgium there) anywhere around the track and earlier this year I was on the phone the whole way round the perimeter road!
I'm on Orange and never had any problem at all in Spa. Proximus sponsor the 24 hour race at Spa (or always did) so perhaps they've got some cells in the area.
I don't ever recall anyone complaining about phone signal which as said above, given the geography is probably unusual.
Didn't have any issues with O2 coverage last month when I was there. Interestingly managed to get free Belgacom wifi in the pit garages for the whole 24 hour race - which was nice.