SIP-addressing is logical, of the same type as URL in HTTP. And the most natural – to use the electronic mail as the SIP address. There are permissible varied parameters by the address: for example, it is possible to indicate the regular city telephone number, additional telephone, and the parameters of modem connection etc.
It should be noted that SIP supports messaging and presence functions. The first ensures the exchange of short messages (like ICQ on the PC or SMS in GSM networks) in the real time, the second allows defining the status of the subscriber, ie., is he on site, not busy etc. (there is also such an opportunity in ICQ). Due to these two functions SIP allows reacting to events as well as sending messages concerning the event.
Тhus, with the help of SIP you can control the access of other subscribers to your number arbitrarily fine, - for example, to set your SIP-environment in a way your mother would always have a chance to get you on the phone but for other relatives and acquaintances there would be limitations. Besides the mother would always call from one and the same number but responding will depend upon your location, mobile phone, personal computer, notebook or, let us say, PDA. If a work colleague calls you during the night, he won’t get you on the phone but you will know that he had called.
Thus, SIP-addresses use e-mail addresses, URL is used for all operations with calls but SIP messages are multimedia and, moreover, objective. Besides SIP supports a special quite powerful language CPL (Call Processing Language) based on XML, which is meant for writing telephone scripts allowing to indicate who when and why calls, what to do if the receiver is not taken or is taken at the wrong place etc. Due to it all it is easy to make the most varied services within the framework of SIP. Similar services can be made by three groups of people: producers of SIP-equipment, service-providers and the very end users. The CPL language is not complicated, so it seems that many will be able to realize the absolutely intricate work scheme of the answering machine: let us say, if the caller dials number 1, he switches unto the subscriber’s home telephone, if 2 – unto the cell phone, if 3 – unto his parents’ telephone etc. But why not to write a script that when the bell sounds a face (photography) of the caller is shown to you? Let us say, a restaurant telephone could instantly give today’s menu on the display, - briefly saying, the possibilities here are limited only by the user’s fantasy.
As all modern ERP-, CRM- and the like systems work after the IP protocol, SIP integrates with them without special problems (apart from H.323, the telephone kind of which hinders its interaction with the majority of attachments).