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][5]) 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][1] shown by Karsten Nohl.  A5/2 is weak and was withdrawn by the GSMA (see [Wikipedia][2]).  At least some providers nowadays support A5/3 (example: [Deutsche Telekom][3] in Germany or [Proximus][4] in Belgium).  A5/3 is based on KASUMI that is also used for 3G.  A5/4 to A5/7 are IMHO not (yet) defined.


  [1]: https://srlabs.de/decrypting_gsm/
  [2]: http://en.wikipedia.org/wiki/A5/2
  [3]: https://www.telekom.com/medien/konzern/209962
  [4]: http://gsmmap.org/assets/pdfs/gsmmap.org-country_report-Belgium-2014-12.pdf
  [5]: http://www.etsi.org/deliver/etsi_gts/04/0408/05.03.00_60/gsmts_0408v050300p.pdf