I think you've misunderstood the use of the word "jammed" in this context ( and apologies for the unintended ambiguity in its useage ) - it means that the networks were so overloaded ( bunged up/jammed/chokka ) with people trying to make calls that the ability to place calls to/from mobiles within the central London area was compromised. The emergency services would also have contributed to this by removing a portion of the available resources for their priority use.

I was trying to contact a number of people in the area on their mobiles for about six hours before I managed to get through to one of them. In that context - it would be extraordinarily difficult to detonate a bomb by mobile - and would also suggest that this bomb was differently triggered ( and therefore differently designed? ) from the bombs used on the tubes - where mobile phones also do not work because there is no "cell" available to route signals either to a mobile or from a mobile. The only portions of the London metro where mobiles work are the parts that go above ground - and these are not the areas that were bombed.