It depends on how closely they monitor the network and how much these people abused it. A steady, slow increase in calls/messages on a site wouldn’t show up in stats unless there was a lot of constant congestion, and even then most telcos these days outsource a lot of their network monitoring and capacity management to contractors that just don’t care.

Plus if they’re using legacy 2G/3G, it’s not the shiny thing that most telco network quality crews care about for customer experience…