China suspected of spying on Americans via Caribbean phone networks

> China appears to have used mobile phone networks in the Caribbean to surveil US mobile phone subscribers as part of its espionage campaign against Americans, according to a mobile network security expert who has analysed sensitive signals data.

This is probably happening via SS7, right? Can someone explain how a non-telco third-party company like Mr Miller's can monitor these signalling messages? (or just point me in the right direction)

How likely is it that this can be independently verified? So far it's allegations reported as facts. I'm prepared to believe him, but it would be better to have solid facts.