The only way to activate your phone's voice dialing feature via the BlueAnt S4 is to say "Phone Commands". If it's calling the last number, then it seems that you are activating the 'redial' feature. All Android 2.1 or above has 'voice dialing over Bluetooth' built-in. However, this feature will only work if your service provider has enabled it.
We suggest for you to contact your service provider for more info on this and also run a search in Google, to see if anyone else (under the same provider) is experiencing the same issue.
Also, if you have other 3rd party voice recognition applications installed on your phone, please ensure that your phone's voice control setting does not default to this app.