Many service provider clients have been sharing that copper facilities that were fine for Internet Access but giving errors on VoIP. Those T1s that were working fine for DIA or MIS (Internet Access), all of the sudden are giving errors on VoIP calls.
Real time traffic like VoIP or video streaming are more sensitive to packet loss than web traffic. Hence, when you run that new VoIP service over an otherwise fine copper service, POOF! trouble.
MOPS testing is always a good idea. TTU (test and turn up) isn't what it used to be with line conditioning. You get what you get (till you call in and complain).