The key task of a mobile phone is to make and receive phone calls. Landlines in the UK are run through the Public Service Telephone Network (PSTN) and the mobile phone must interface into this network to allow calls to and from a mobile phone and a landline, as well as mobile to mobile. The role of the exchange is taken by the Mobile Switching Centre (MSC). In the PSTN the landline is always connected to the same exchange, but this is not the case with a mobile phone; hence the added complexity with mobile phone telephony.

Making a call to a mobile phone:

• The phone calls the mobile through the PSTN
• The PSTN realises the number called is a mobile and a connection is set up to the Gateway (GMSC).
• The (GMSC) receives the call and queries, using the MSISN, finds the phone's HLR.
• The HLR finds the IMSI from the MSISDN and hence the serving VLR / MSC for the receiving phone. The HLR get the phone's Mobile Station Roaming Number (MSRN); the number used by the VLR / MSC to identify the phone, which is passed to the GMSC.
• The GMSC routes the call to the correct VLR / MSC using the MSRN.
• The MSC knows the location of the called phone in terms of the BSC, base station controller, it is connected to.
• The connection is made to the BSC and then onto the relevant BTS (or mast) which the phone is connected.
• A message is sent via the control channel to all phones connected to the BTS. Only the phone with the correct IMSI will respond. A dedicated channel is allocated to the call.
• When picked up the call connection is complete
• The local MSC controls the phone call and will terminate when either party cuts the call. Additionally messages indicating the receiver is busy, or rings out, are generated here. So calling a foreign country leads to messages from that country in their language.

Calling from a mobile phone:

• The mobile phone calls another phone using a random control channel to request a phone call from the BSC.
• The BSC, on hearing the request to make a phone call allocates a unique channel through which the call is set up.
• The MS sends a request to the MSC to make the phone call. The MSC checks if the mobile is authorised to make such a call, analyses the destination and sets up to initiate the call.
• MSC asks the BSC to allocate a channel for the phone to communicate.
• BSC allocates the channel
• The MSC calls the GMSC to request the call is connected.
• The call is connected through the PSTN