One way to improve voice recognition when making calls is to be as descriptive as possible including saying the full name and the source of the number (ex. mobile, home, work..).

For example, when trying to call John Smith's cell phone, instead of just saying "call John", try "call John Smith Mobile" and you may have better luck in getting the software to recognize you. Calling John Smith at the office would go "call John Smith Work".

And, if you're using your Bluetooth and you're outside just know you're gonna have to cup your hand around it when you talk. *think old school radio announcer* :)