Answer:
The mid 1990s
Explanation:
This was the closest I could get to a real answer. I've scoured the internet. This is certainly a tough one. In any case, it might serve you to know that  registration for the Stingray was first filed in August 2001 (source below). A Stingray device is the most commonly used cell site simulator device.
Source: https://arstechnica.com/tech-policy/2013/09/meet-the-machines-that-steal-your-phones-data/
