What is "openbts"

OpenBTS (Open Base Transceiver Station) is a software-based GSM access point, allowing standard GSM-compatible mobile phones to be used as SIP endpoints in Voice over IP (VOIP) networks. OpenBTS is an open-source software that was developed and is maintained by Range Networks. The public release of OpenBTS is notable for being the first free-software implementation of the lower three layers of the industry-standard GSM protocol stack. It is written in C++ and released as free software under the terms of version 3 of the GNU Affero General Public License.