They knew where the front line was, they were trying to learn where the Ukrainian forces were positioned along that front line.

Right, so back to the question - why not just monitor front line cells for activity, which i think can be done without making calls?

I'm guessing: they needed a way to distinguish the troops from the civilians living in the area. By triggering a mass amount of calls to frontline soldiers, they made their positions light up in front of the overall background.