Speed alone isn't the only consideration. Packets have to arrive kinda in order and at least in time to be put in order. So try a VOIP test that also measures "quality". Google and a bunch will come up. Here's one I've used before.

MySpeed VoIP connection test, VoIP test, test VoIP performance, test VoIP quality

There's a box in the middle that say "Click to start MySpeed".

There will be a report that rates the quality of the connection and tells what stuff is off. On Fios I get all green ratings. On Time Warner cable I used to sometimes get some yellow, but not enough problems to interfere with calls.