Interesting question !!
I can check tomorrow at work, but as far as i know :
the electromagnetic fields from the powerlines will not impact the propagation from the GSM antenna… the frequency should be about 50Hz (or 60Hz ? where do you live ?) anyway, it is very far from 900MHz !
the cables themselves can be considered as obstacles, which will create diffraction… but even ten cables in a row, it is still a very small amount of the signal that will hit them. So the amount of diffracted signal might be neglictible.
Furthermore, the cables are not really in the same scale (in terms of size) as the radio waves (gsm900 = 32cm, dcs1800 =16cm, a cable diameter = ?? cm) , so i’m not even sure there will be diffraction…