While I'm awed by Harald Welte, I don't think this project makes a lot of difference.
Cell phone towers are mass-produced and are pretty cheap. You probably can't make anything much cheaper as it is on large scale. On small scale it might be possible, but it'll still probably be cheaper to buy a nanocell.
And while GSM is nice, it's now clear that the 'path forward' is 4G which is going to use VoIP over an IP data network.