When an SMS or a call goes out on GSM 2G, the tower starts the exchange by sending a ping request to the receiving telephone. Once the receiving phone answers, the SMS or call goes through. These jammers work by responding to the tower ping before the right phone can. This is not technically "jamming" but the effect is the same. The researchers didn't design the hack to read stolen communications, but there's no reason they can't.
Fortunately, there are a few captures. So far, it looks like Motorola phones are the only ones that can be turned into these wifi jammer. In addition, the jammers only work on GSM 2G networks and can only block calls and texts to phones from the same provider. A T-Mobile interference can therefore only block calls to other T-Mobile phones, and CDMA carriers like Verizon and Sprint are safe.