Simplify the following text for a Grade 7 native English-speaking person.
Original: A full GSM ( GSM-900 and GSM-1800 ) , CDMA and TDMA mobile phone network is in place through out the MTR system of stations and tunnels allowing passengers to stay connected underground .
Simplified: A full GSM ( GSM-900 and GSM-1800 ) , CDMA and TDMA mobile phone network is in place through out the MTR system of stations and tunnels . Passengers can stay connected underground .