Our customers use Thirdlane Multi Tenant PBX 
Thirdlane Multi Tenant PBX was one of the first (if not the first) Asterisk-based Multi Tenant products, first released in 2005.
Thirdlane Multi Tenant PBX uses Asterisk realtime, provides advanced features such as redundancy, clustering, and also offers integrated Call Center option. Hundreds of small and mid-size ITSPs worldwide use Thirdlane.
Thirdlane is actively maintained, last major release was in October 2013. You should definitely check it out here: http://www.thirdlane.com/products/thirdlane-pbx-mte