I read about OpenBTS it's really amazing... but I was wondering if we can use it to build phone-to-phone provider-less network.
Any clues or experiments are really appreciated.
One thing to be aware of is that open BTS is 2G/GSM only - i.e. it does not support 3G/UMTS.
This may or may not be important to you depending on what you would like to achieve.
There does appear to be some discussion on adding this functionality in the future (i.e. building an open node b/RNC effectively) but it will be tricky as the authentication mechanism used in 3G requires the network owning the SIM to provide authentication data for even the most basic communication.