Value-added service is an additional communication service developed by using the resources of public telecommunication network and other communication equipment, and its realized value increases the economic benefit or functional value of the original network, so it is called telecom value-added service. Value-added telecommunications services are relative to basic telecommunications services. Telephone, telegraph, telex, fax and data transmission directly provide information transmission services to users by using public telecommunication network, which is called basic telecommunication service. Its basic feature is that the content and form of the letter will not change when it reaches the recipient. Basic telecommunications services are required to be generally provided to the public.