PABX or KeyPhone system are very old technology and it is stable. So, if your company is using one of it, why change? If it serves you well, then we do not recommend you to change your system, but we strongly recommend you to add VoIP functionality to your system to enjoy the benefit of VoIP system.
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.