Most likely CW1308 but,I'm not sure how many pairs you needed, it's available in various 2,3,4 & many others. Just watch when you buy the cable, quailty varies, look for conductor in the cable which is pure solid copper and not copper clad steel or tinned copper (with cheaper versions) http://www.cablemonkey.co.uk/telephone-cable/9488-cw1308-internal-phone-cable.html Not familar with the BT versatility ccu, in the link below, under support there is Free Phone support number for setting up, they will tell you how many pairs you will need. https://www.pmctelecom.co.uk/bt-versatility-ccu-multi-line-phone-system
I ve used a normal phone wire and cut one end and put the two wires into a and b in unit but I can phone out I keep getting busy sign and nobody can phone in as they hear as though phones ringing but no one answering
Have a look at the manual, it's in downloads in my link. Your normal phone wire, will most likely be CW1308, sadly I have no experience with the BT versatility ccu, or setting one up. A good place for advice is the Kitz Forum, they have a Telephony Wiring + Equipment sub forum, place is full of OR/BT engineers,very helpful lot. http://forum.kitz.co.uk/