latency and bad connection

I use a small local VOIP provider (VOIPP). The sound quality is good; my problem is misconnections, of three types, all of them sporadic, they come and go spontaneously, no action required:
1. Few seconds-minutes-hours of no dial tone (and no telephone lights).
2. I answer a call and hear nothing, then a busy signal. The calling party hears nothing at all after a couple of beeps.
3. Often an 800 or a calling card access number or something similar, that is not supposed to ever be busy, IS busy. Sometimes it’s not busy at my second attempt,sometimes I have to use my cell phone.
This gnome company says that the router (which they do not provide) is at fault, or that my cable line has high latency. I checked both upload and download speeds and packet loss – I am near the high end of the scale.
I think that misconnections (they often occur at early morning hours) are ALWAYS VOIPP’s fault.
Am I right?