All you need is a GSM Phone, any SIM Card and the OpenBTS software to create a tiny GSM network using Orbit Hardware. 

The first thing you should ask yourself is -> Which SDR's does OpenBTS support for GSM? -> http://openbts.org/w/index.php/RadioIntegration

I have had all of my success with the N210, which was available on Orbit Sandbox 3 at the time of writing this article.
How did I know the N210 was available on SB3? -> If you can find the "Orbit Online Scheduler" then you should be able to find the "Orbit Status Page", you can ask the "Orbit Status Page", "On which nodes is the N210 installed?"

Once you know which box is hosting your target SDR, the next thing you should do is to verify that the SDR is working on your target box.