Security experts eavesdropped and geographic tracked a US Congressman only using his phone number. The activity was authorized by the US Representative Ted Lieu in order to demonstrate how much we are vulnerable. The security issue in the SS7 signaling system could be exploited by criminals, terrorists and intelligence agencies to spy on communications. The flaws discovered by the German researchers are actually functions built into SS7 for other purposes such as keeping calls connected as users speed down highways, switching from cell tower to cell tower that hackers can repurpose for surveillance.”]
Source: https://securityaffairs.co/wordpress/46473/hacking/ss7-protocol-surveillance.html