In some cases Voipo or frankly any SIP provider can be a fail. They are better than most but what you are reporting could almost be any SIP or voip provider. I agree with you on one item, regarding routing they could or should open a dispute with the carriers not routing calls correctly.

It's not there switch or technically not there problem -but any provider should seek to rectify the issues with the problem routing carrier and can do so fairly easily with call examples.

Unfortunately voip is a take it or leave it technology, when it works its fantastic. When it doesn't its a pain and often providers don't do as much as they possibly could. I have found VoiPo to be a very good provider. Raise your issues with a ticket, should you not like the responses its probably best to leave