Now it's time to create a SIP trunk. The best thing to do is use one of the supported VoIP providers. You can use anyone if you get the correct information but, if you have problems, 3CX won't be able to help you out.
Just like setting up the PSTN line, you can get to the VoIP provider wizard in the same manner. I'm going to click Add VOIP Provider Wizard on the top menu bar. Pick the method you are most comfortable with.
The first thing you want to do is see which VoIP providers are supported and set up an account on their website. While the 3CX wizard is quick and easy, it cannot create your account with an ITSP. As I'm in the US and Callcentric lines are free (as long as the call is to another Callcentric customer), I am going to use a fake one to get us started. Go ahead and set up the account with the provider of your choice, and get your account details. Once you get the account information, it's time to move on and enter it into 3CX so that you can make calls.