I doubt this could be the problem as you are already running ovet two separate ISP links but i was wondering about packets arriving out of order because you have multiple links. Out of order packets can significantly affect voice packets so i wonder if this could be an issue.
Like i say, unlikely, but just suggesting possible causes.
Just a clarification, all 3 links are from same ISP.
As far as your doubts goes over packet arriving out of order, even I doubted the same, however I'm still wondering how to check that and what action as such can be taken to rectify out of order packet arrival issue.