CalypsoBTS - Osmocom-BB GSM Basestation
More about visit : https://osmocom.org/
Install steps : https://pastebin.com/Gi9jx6Tz
I have add osmo-bts-trx-calypso.cfg that I had forgotten in my previous pastebin : https://pastebin.com/pVmj0PZc
Sound : Rez - Monday