Cellular phone systems first try to route a call to the last cell which was in contact with the phone. If this fails the system tries to locate the phone to other cells and then tries to divert to voicemail.
The system pages the mobile in EVERY cell of the Location Area at the outset. When not in a call, the system does NOT know which cell a mobile is in, it only knows the Location Area. Only when the mobile responds to the page should the ring tone be played to the caller (less networks which offer ringback tones). If the mobile does not respond to the page, the network can send the call to VM (if the subscriber has paid for it). When it hits the VM platform, there may be a second or two of ringing before the call is answered by the the system.