If a MS Roaming in a MSC Coverage Area,defnly MSC Knows the location ( ie LA-Location ) of the MS.
This is default system design.Then only MSC Can Page the LA for any MT Calls.
Then in case of MO-Calls the Call setup messages will carry the Location informations ( CGI,CI..etc..etc ) which will be used for some other puposes like Charging,Location Based Routing,Emergency Calls…ertc…etc.
If MS is in Busy Condition you can trace him out as explained by others.
Using CELLNUMBER ( ie Cell ID ) Emergency Call routing/Charging/Routing also can be done differently.But depends upon the system.