It's possible given the number of extensions that it's a REN problem
http://www.britishtelephones.com/ren.htm
http://www.wppltd.demon.co.uk/WPP/Wi...tallation.html
"REN measures the load a device places on the line when ringing. A normal BT line will support a REN of at least 4, in other words at least a total of 4 phones/fax/modems should work on any line so long as their REN figures added together don't exceed 4. The REN is normally found on a label at the base of the machine (near the green approval symbol)."