TMS stands for ‘telephone management system’ and is simply the piece of software that connects to your phone system and allows you to manage, analyse and track the calls your team takes and makes, the cost of your telephony, and create and customise call queues. 

PBX is ‘private branch exchange’ and is the private phone network that you use inside your company. The users of the PBX can make and receive calls within the company and externally to/from clients using different connection types such as VoIP (voice over IP). PBX also allows you to have more phones running than actual phone lines available. PABX which stands for private automatic branch exchange nowadays is the same as PBX as everything has become automated in the last couple of decades. 

‘Session Initiation Protocol’ is the internet protocol that is used to initiate, maintain, and then terminate real-time sessions such as voice and video calls. It is one of the main applications used within VoIP telephony.

‘Voice over Internet Protocol’ is the fundamental protocol used for internet-based telephony as it is the protocol used to provide phone services over the internet.