Quoting from the article:
“The government would then use that key to decrypt voice data separately captured by the subject’s ISP (likely a mobile provider in this case).”
They also could just capture the data from the subjects' routers, assuming they can be hacked (which for most people is probably the case).
> Force Facebook to retain the session key to the suspect’s conversation and turn it over to the government. The government would then use that key to decrypt voice data separately captured by the subject’s ISP (likely a mobile provider in this case).
They can wiretap the data lines, it's not a problem for the gov. The problem is decrypting that data.