United States v. Ramirez, 112 F.3d 849 (7th Cir. 1997), involved the investigation of a drug-related conspiracy that "straddled the border between two states, Wisconsin and Minnesota." Id. at 850.
A judge in the Western district of Wisconsin authorized the interception of cell phone calls, ultimately from a listening post established in Minnesota. In holding that the interception was lawful regardless of where the cell phone itself was located, the opinion of Chief Judge Richard Posner announced, "We do not think that the location of the phone affected the legality of the tap," id. at 852, and then explained why the physical location of a cell phone does not have the significance that the physical location of a landline telephone would have in a traditional wiretapping case.
The legislative history of Title III suggests, unsurprisingly, that "mobile interception device" was intended to carry a broader meaning than the literal one. This history describes the term as applicable "to both a listening device installed in a vehicle and to a tap placed on a cellular or other telephone instrument installed in a vehicle." ... And a tap is not placed in the telephone handset itself; it is attached to the telephone line a some distance from the handset. The listening post in this case intercepted transmissions between microwave towers that relayed cellular phone calls, and so was analogous to a tap affixed to a telephone cable outside the subscriber's premises. The emphasis in "mobile interception device" falls ... on the mobility of what is intercepted rather than on the irrelevant mobility or stationarity of the device. (Id. at 852-53.)