The services provided by professionals in particular field. For example, a company may charge a certain sum of money for the certain tasks their consultant accomplished for the company who ordered the service.
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.