HI Kris Buytaert.
I'm Colombian's Linux Developer. I have developed a system called GNEAX (http://www.admatel.com). with It software the Organizations can obtain telephonic control.
About the MySQL cluster NDB: Figure out two or more PBX in severals Organizations that want integrate databases of calls and have the same information to.
About Openmosix: Figure out severals computers with linux and balancing Apache loads. In General, the databases are big because contain very information about the calls.
This case is EMTELSA-Manizales-Colombia that has several builds with different PBX .