First of all you need find where the problem is: CO, Internal wiring or PBX. Test CO first, disconnect this pair from any internal wiring and protector if you can. Connect your set strait to the pair from CO and call (from cell phone) this TN as many times as needed to make it trip. If one of your calls trips – you have CO problem, call them and explain the problem. If no trips after 30 or 50 calls start testing internal wiring. Disconnect this TN form PBX, connect your set to it and make 50 calls- if one of them trips you found problem, if not - check your PBX.
