I know these zero length calls and I hate them. Two days ago I got 16 of them because there were problems between my ATA and Sipgate. Also I got some between Voxalot and Tpad.
All those problems still make me shy away from PBXes. I suppose it will bring more of those problems to me. I use about 20 different VoIP services with their respective phone numbers. My idea is to concentrate them all in 1 voxalot account which would then be ringing a PBXes account. People would then call me on my German number from Sipgate or one of the dozens of inbound numbers by Tpad. For outbound calls I would use Voipstunt's cheap prices.
But I hear about so many problems between the companies involved in this scenario so that I still don't want to set it up. Your latest message makes it even worse.