Just because you have a 300 pair coming in to your building doesn't mean you have all 300 pairs for your use. Many times telco is just tapping off the main trunk going by your building - so you're probably getting dial tone for other telco customers in the area on the 'undocumented' pairs coming in. That said - what telco should be providing you is what's called RJ21X terminations (usually a 66 or Krone block with an orange cover in Verizon territory) that you the customer connect to, and they should tag those lines that you're being billed for. Anything else isn't for you to use - they're there for the phone company when you need to add new circuits or move you off to different pairs.
Any good telecom manager type of person should know what he's paying for and what's actually being used. Check your billing, check your RJ21X's, see where they're terminating to.