I know that pylons and phone masts are not exactly the same,but....
When we ere looking for an apiary site about 5 years ago,the site which ticked most of the wish-list was directly under a High Tension line (10,000V I think) and perhaps 50yds from the nearest pylon.
After some agonising over the possible effects on the bees,the site's other advantages took precedence over our concerns about electro-magnetic field.The bees have done well and are no different in behaviour from other bees kept by my fellow-apiarists elsewhere,despite the dire warnings from some quarters about pylons causing "angry bees".
The fields emitted from both mobile phone masts and from high-tension lines obey the inverse-square law,so unless you are in REALLY close proximity (feet rather than yards) I don't suppose there is much effect.