According to GSM 02.07 B.1.26, there should be a Ciphering Indicator in the ME to allow a user to detect if ciphering is not switched on. The Ciphering Indicator can be turned off by the network operator clearing the OFM (Operational Feature Monitor) bit in the "administrative data" field of the SIM (see GSM 11.11, 10.3.18)
Usually the Ciphering Indicator is turned off, at least in those SIMs I have seen so far. And you usually cannot modify the administrative data in the SIM. But would a phone actually display something if the Ciphering Indicator is enabled and ciphering is not on ?
To find this out I used a Test SIM which allows modifying all fields of the SIM. Such a Test SIM is for testing handsets, you cannot use it with your official GSM network. The OFM bit was set to one to enable the Ciphering Indicator. To simulate encrypted and unencrypted calls I used a Racal 6103E GSM Testset. I could have also used OpenBSC, however with the 6103E switching between encrypted and unencrypted calls was faster, it just required the press of a button.
Here are the results for some phones I had access to, Yes means that I noticed a difference between encrypted and unencrypted calls and what this difference was and No means that I noticed no difference:
- Nokia 3310: Yes, Open Lock Symbol during Call
- Siemens M55: Yes, *!* Symbol during Call, regardless of OFM bit
- Mitsubishi MT-040: No
- P1300: Yes, Cx Symbol during Call
- Motorola C123: No
- Motorola W156: No
- Netzing NE110: Yes, C* Symbol during Call
- BlackBerry 9000: No
- BlackBerry 9520: No
- BlackBerry 9700: No
- Sony Erisson K800i: No
- HTC Touch Pro: No
- HTC Hermes: No
This is by no means a detailed investigation however it is at least some indication that a lot of phones do not care at all about the Ciphering Indicator, probably because it is usually turned off in the SIM anyway.