1. Deering, S. & Hinden, R. 1998. RFC 2460 - Internet Protocol, Version 6 (IPv6) Specification. In: IETF - Internet Engineering Task Force [online]. Available at: https://tools.ietf.org/html/rfc2460 [Accessed: 1 November 2020];
2. Donovan, S. 2000. RFC 2976 -The SIP INFO Method. In: IETF -Internet Engineering Task Force [online]. Available at: https://tools.ietf.org/html/rfc2976 [Accessed: 1 November 2020];
3. Ecma International. 1998. Standard ECMA-133: Private Integrated Services Network (PISN) -Reference Configuration for PISN Exchanges (PINX) [online]. Available at: https://www.ecmainternational.org/publications/standards/Ecma-133.htm [Accessed: 1 November 2020];
4. Ecma International. 2001a. Standard ECMA-142: Private Integrated Services Network (PISN) - Circuit Mode 64kbit/s Bearer Services - Service Description, Functional Capabilities and Information Flows (BCSD) [online]. Available at: https://www.ecma-international.org/publications/standards/Ecma142.htm [Accessed: 1 November 2020];
5. Ecma International. 2001b. Standard ECMA-143: Private Integrated Services Network (PISN) - Circuit Mode Bearer Services - Inter-Exchange Signalling Procedures and Protocol (QSIG-BC) [online]. Available at: https://www.ecma-international.org/publications/standards/Ecma-143.htm [Accessed: 1 November 2020];