Journal of Information Technology Review Todorka Georgieva and Borislav Necov 13 4 2022 https://doi.org/10.6025/jitr/2022/13/4/92-98 https://www.dline.info/jitr/fulltext/v13n4/jitrv13n4_2.pdf We in this work, have described the VoIP systems that use the PSTN and software. The interfaces in the voice protocols and the modules contained in them are tested. Besides, we have used the voice gateways to describe the process. The experimental trials and the results are presented.