The Service Provider Gateway system is a provisioning gateway in a telecoms' network used as part of wholesale line rental to allow telecoms operators to control provisioning on the Openreach network.
SPG as it is more commonly referred to has the ability to check an address for a standard BT line and then the agent using the system can order a new BT line, order the start of a stopped BT line, if required by the customer.
SPG cannot detect Virgin Media lines, or lines that are on an LLU (or local-loop unbundling) network such as TalkTalk, Tiscali, or British Sky Broadcasting.
References
Service Provider Gateway Wikipedia(Text) CC BY-SA