IMS refers to "IP Multimedia Subsystem". It is a standardised network used for delivering multimedia services, such as voice and video calls to the users.

IMS can be integrated with 2G, 3G, 4G, and 5G mobile networks, providing a unified communication platform for mobile subscribers.

While 2G/3G networks are having a Circuit Switched Core Network (CS Core) that can be used to deliver voice calls to the users, the 4G and 5G networks are having only a packet core network.

Therefore, IMS integration is essential in case of 4G and 5G networks, in order to be able to deliver voice and video calls to the users by using VoLTE and VoNR respectively.