Typically, your company have a device that accept phone lines from telecommunication provider. Telecommunication providers refer to any fix line phone number provider. In Malaysia, it refers to Telekom Malaysia, one and only fixed line provider in Malaysia.
The phone line arrive at your company via 3 interface, which is with RJ 11, RJ 45 or Coaxial line. Anyway, lets make it simpler, if your phone lines are less than 30 numbers, most probable you are using RJ 11 and if you have 30 or more lines, you are using RJ 45 or other.