Calls over GSM might be encrypted. The phone and the base station initially agree whether to encrypt at all and if which algorithm to use. The phone sends a list of supported ciphering algorithms to the base station. The base station sends a 'Cipher Mode Settings' element (see GSM 04.08 Chapter 10.5.2.9) to the phone. This element holds one bit that marks if encryption should be used. If encryption should be used, there are three bits that define the used algorithm:
0 0 0 cipher with algorithm A5/1
0 0 1 cipher with algorithm A5/2
0 1 0 cipher with algorithm A5/3
0 1 1 cipher with algorithm A5/4
1 0 0 cipher with algorithm A5/5
1 0 1 cipher with algorithm A5/6
1 1 0 cipher with algorithm A5/7
1 1 1 reserved
Speaking of the security, there was a practical attack against A5/1 shown by Karsten Nohl. A5/2 is weak and was withdrawn by the GSMA (see Wikipedia). At least some providers nowadays support A5/3 (example: Deutsche Telekom in Germany or Proximus in Belgium). A5/3 is based on KASUMI that is also used for 3G. To my knowledge, A5/4 to A5/7 are not (yet) defined.
