Obstructions – such as buildings, vehicles, trees, hills and building materials – can all reduce the signal level available for your mobile device. Indoor mobile coverage is susceptible to interference from the building you are in, and just as TV sets sometimes need external antennas to improve reception, there are cases where the use of an external antennas can make a significant difference to the performance of a mobile device indoors.
Telstra Coverage Map
OZ Towers click here To Check your mobile towers / coverage / frequencies in your area.Worldwide GSM Frequencies for Mobile Providers By Country
Click Here to find the Country Frequencies you require whilst visiting another country.