Making Calls

Calls to Mobile Networks
Making a call to a mobile network involves keying in the called subscriber’s
MSISDN (mobile subscriber’s ISDN) number. This number routes the call to the
called subscriber’s home network via a gateway mobile switching center (GMSC).
Here, signaling interactions with the home location register (HLR) occur. The
HLR, in turn, interacts with the visitor location register at the mobile switching
center at which the called subscriber is currently registered.
Eventually, although it takes very little time, new routing information is sent
back to the GMSC to replace the subscriber’s MSISDN number. The new information
allows the call to be routed to the network in which the called subscriber is
currently registered and, more specifically, to the serving MSC (either at home or
abroad [roaming cases]).

Calls from Mobile Networks
For outgoing calls from a mobile network, the serving MSC acts as a local exchange
for the subscriber that is making the call — with the call being routed directly from
the serving MSC (rather than having to route back through the subscriber’s home
network if abroad).

In all cases, however, the call is in accordance with any service information
being held in the VLR (having been previously sent from the subscriber’s HLR
when first registering at this MSC). This may include such restrictions as call barring
or advanced interactions required for prepaid subscribers