GSM phones normally encrypt the radio part of the call, however the handset is instructed to do this by the base station. In India all GSM calls to/from the base station are un-encrypted, as that is the law. Once the call passes from the base station onto the fixed network it will not be encrypted, as the GCHQ man says.
One technique used by the spooks is to insert a dummy base station. This then instructs the handset to turn off encryption until further notice. So long as the spook remains within range they can listen to both legs of the call. This kit is impossible to buy unless you are approved by HMG or the US Govt.