The U.S. government concluded within the past two years that Israel was most likely behind the placement of cellphone surveillance devices that were found near the White House and other sensitive locations around Washington, according to three former senior U.S. officials with knowledge of the matter.
But unlike most other occasions when flagrant incidents of foreign spying have been discovered on American soil, the Trump administration did not rebuke the Israeli government, and there were no consequences for Israel’s behavior, one of the former officials said.
The miniature surveillance devices, colloquially known as “StingRays,” mimic regular cell towers to fool cellphones into giving them their locations and identity information. Formally called international mobile subscriber identity-catchers or IMSI-catchers, they also can capture the contents of calls and data use.
Israel accused of planting mysterious spy devices near the White House
Bit about imsi catchers
https://www.libertyhumanrights.org.uk/sites/default/files/Explainers-IMSI_1202.pdf
Imsi is not the same as the telephone number, its a unique number which identifies the user to the mobile network operator. I assume the imsi's captured are then used to help spies work out which mobile phone calls to decode and monitor.
"When operating in active mode, the Stingray device mimics a mobile phone base station in order to force all nearby mobile phones and other cellular data devices to connect to it."
Stingray phone tracker - Wikipedia