Take the 2G GSM network as an example
The mobile station calls and accesses the base station. The base station connects to the BSC. The BSC connects to the MSC. The MSC goes to the HLR where the called number is located to find the current number. The MSC (VLR) where the call is located, find the MSC that will be connected to the called number in the future, and the MSC initiates a call to the user.
The process is roughly like this, but of course it is not that simple.