The KnowRoaming SIM is a global SIM card so when making an outbound call you will need to dial using the international dialing format.
[ + ] [ country code ] [ phone number ]
So for example, if you wanted to call a Canadian or US phone number you would dial:
+ 1 555 666 7777
This format is used at all times and does not depend on where you are or where you're calling to. So even if you're roaming in the USA and you want to call a USA phone number, you would still use the international dialing format.
Things get a trickier for our North American customers roaming the rest of the world because we're not used to something called a trunk code. It's the 0 in front of someone's phone number. So someone from the UK may say "call me at 0751234567". What you would need to do is drop the trunk code (0) and add the plus and the country code in front. You would dial the number like this:
+ 44 751234567
So anytime there is a 0 at the beginning of someone's phone number, just make sure to drop it then put the plus and the country code in it's place.
For a full list of all country codes, please see the article here.