Never assume that voice calls are confidential (like fax or email), especially when calling internationally where some countries' phone operators have no encryption security in place at all. Check your signal, calls on 3G are more secure than 2G but often falls back to 2G when 3G is unavailable.
Keep your phone safe and do not leave it lying around. Skilled attackers can take just a few moments to install a malicious program, compromise the security of the SIM card or install a special battery with a bug in it, all of which can later be used to help intercept calls.
Use and protect your phone and voicemail PINs in the same way as your bankcard PIN. Never leave confidential messages in voicemails or send confidential texts. Texts in particular are easy to read on the phone and mobile phone voicemails can often be accessed from any phone with the PIN.
Be vigilant to prevent malicious software on your phone. Be wary of texts, system messages or events on your phone that you did not ask for, initiate or expect. Turn off Bluetooth if you are not using it. Consider anti-virus / anti-malware software, and if you strongly suspect your calls are being listened to then turn off the phone when you don't need it and remove the battery as an extreme precaution.
Use voice call encryption software on your phone to secure your sensitive calls that works worldwide and is as easy to use as making a normal phone call.
If you have no alternative (such as using encryption software) and urgently need to discuss confidential matters over a mobile phone: