Most phones do issue a warning if ciphering isn't enabled. On some you may be able to force it to require it. But keep in mind that this is only applied on the

Most phones do issue a warning if ciphering isn't enabled. On some you may be able to force it to require it. But keep in mind that this is only applied on the radio interface anyways (and GSM encryption is so broken you shouldn't be relying on it anyways). If you want end to end encryption of your calls you will need to use encrypted VOIP over your data connection.