## 1 Introduction
Asterisk, by virtue of being an open source system created by a community of developers, is ideal for rapid development that will ensure compliance with such rapidly changing standards. Asterisk does not focus on profitability analysis or market research. It evolves in response to whatever the community finds interesting or necessary. Compared to the implementation of traditional PBXs in offices, using Asterisk PBX has the following advantages:  
* A single network infrastructure, which Asterisk has, allows a wider range of services to be received both for office maintenance and business services, reducing the cost of technical staff;
* In traditional PBXs, the maximum message recording configuration is limited to 32 h of recording, whereas in Asterisk, the number of voice messages can be limited by the administrator and depends on the capacity of the hard disk;
* Asterisk has a powerful ability to create an interactive voice menu;
* Asterisk has a wide forwarding functionality, which includes forwarding by time of day, presence at the workplace, simultaneous forwarding to several phones, individual forwarding algorithm, etc. ;
* Fax reception is carried out to all numbers with automatic sending to e-mail;
* Asterisk runs on operation system Linux, and it is possible to combine the functions of a mini-PBX and an Internet server. For example, when using an E1 channel, the data part and the voice part can be separated. Thus, through the same channel, both voice services and access to the Internet and other services from the same provider can be received [22; 23; 24]. The phone system, based on the Asterisk PBX, has great features that are not available in a traditional PBX with switching. Voice mail, conferencing, call queues and agents, music while waiting, and call parking are just some of the features provided by Asterisk [25]. The IP network architecture based on Asterisk PBX completely solves the problem of organizing multiple access from one system to another [26; 27]. But, despite the many advantages of the Asterisk PBX, the problem of secure data transmission in the network, especially when it is scaled when the PBX acts as an independent service system, remains not fully understood [28; 29; 30; 31; 32]. Most research is currently focused on protocols and encryption [33; 34; 35; 36], as well as the quality of the transmitted voice traffic [37; 38; 39; 40]. This does not eliminate the main threats as the attackers are currently focused on something else [41]. The availability of Asterisk from the Internet is one of the main threats to network security. When designing an IP network based on programmable exchanges, it is necessary to take into account the network architecture and system configuration to ensure secure data transmission [42; 43; 44]. To increase the level of security at the first stage, it is necessary to design the network correctly, and then it is necessary to protect data transmitted over open communication channels from interception and listening [45; 46; 47; 48; 49]. From the review, we can conclude that due to the widespread use and implementation of programmable PBXs in small and medium-sized businesses, in particular Asterisk, the topic of developing a real network based on IP PBX Asterisk is relevant. In addition, an important component is to ensure the security of the designed network using modern tools and methods. The details of designing a real network, presented in this article, allow users to avoid errors when installing and configuring the Asterisk server. We decided to use hosting, with one server located in Germany and the other in the Netherlands. This approach shows the flexibility of Asterisk, i.e., that users can deploy a network even without their own equipment, and also shows the importance of ensuring the security of transmitted traffic in such design conditions. The article uses the TLS protocol to protect the transmitted traffic. The structure of this paper is as follows. Section 2 presents a proposed systematic methodology for developing a secure IP network.