PCMs or Phase Change Materials are used in B. T. Shelters in a few diferent ways.
1. Back-up to air conditioning in case of power failure.
2. Free cooling with-out air conditioning and maintaining the Shelter temperature at less than 35C or 40C etc.
3. Latest-PCM is offering trials for PCM based back-up for eliminating diesel generator in case of power failure. You will find a write up on PCMs for BTS (B. t. Shelter) at pcmenergy.com
4. In India PCMs are extensively used in Airtel (Bharti) shelters. Tatas are in a trial stage and Reliance may follow the suit.