One of the options for VoIP telephony is the SIP Trunk service. At the same time, by connecting the PBX (mini-PBX) of the client to the DELTA-Telecom telephone server via the SIP trunk, access to the network of other operators (local, mobile, intercity and international) is provided. There are no restrictions on the number of channels in the SIP trunk service, this service is more convenient for offices and enterprises with a relatively large internal telephone network and call center.