We've been talking a good bit about unlocked phones (the network unlocked kind) and went over some of the benefits of them, and why you might want to have one. Hopefully, it has answered some of your questions about it all.
Now to talk about how to network unlock your phone. All of those mentioned benefits and reasons to have an unlocked phone might have struck the right chords, and you'll need to know how to get it done.
Is your phone already unlocked?
This is where we start. A lot of new phones are already SIM unlocked, even if you bought it from a carrier.
Some phones are sold unlocked, like a Pixel phone or a phone you bought from Amazon or eBay or the company that made it as an "international version." Not every company wants to sell locked phones, but some companies actively advertise themselves as an unlocked phone solution.
Your phone might already be unlocked, so check that before you do anything else.
Phones bought from Verizon are also usually network unlocked. Verizon uses a network configuration without a user-accessible SIM card for voice and 3G data (CDMA) but for the 4G LTE connection they use a standard SIM card. Because of the agreement with the U.S. government when Verizon leased the spectrum they use for LTE, they are required to not block any device from using that part of their network. When that happened, they just started selling unlocked phones. Sprint isn't covered by these regulations, but they also sell some phones unlocked. Others will need to be unlocked the "normal" way.
It is worth noting that while a phone from Verizon or Sprint may be SIM unlocked, that doesn't guarantee it will work with AT&T or T-Mobile or an MVNO. Some do, some don't, and some need a bit of hackery because the APN is locked.
Most phones you buy at the AT&T or T-Mobile store are going to be SIM locked to their respective network. The same goes for Canadian carriers.
Your carrier can unlock your phone
Usually, this is the best way to get your phone SIM unlocked. This is how it works.
U.S. and Canadian laws say that a carrier has to unlock your phone once you meet certain requirements. To get the process started, tell your carrier that you need to network unlock your phone. If you've satisfied their requirements, they give you a code that will do just that. Place a SIM card for a different network in the phone and it will ask you for that code. Enter it (sometimes a reboot is needed) and the SIM should be recognized. The SIM card you use for unlocking doesn't have to be on a line that is yours. You can borrow one from a friend or even walk into your new carrier store and borrow one there.
You might even be able to do all this through a carrier app installed on your phone, so give that a check if you'd rather not call anyone.
You might have to enter network settings (APN settings) for the new network, but usually "big" network operators have an APN in the system software that will automatically be used. If you need to set a new APN, the people you bought your new SIM card from can help you. Don't worry, it's pretty easy.
It really is this easy. But your carrier might not want to unlock your phone. Each has rules about who is "qualified" to get an unlock code, and if you're still under any sort of finance contract for the phone, they may not want to give you one. While these rules are different from carrier to carrier, generally you'll have to have an account in good standing, have had the phone for a certain period of time, and what they consider a good reason for network unlocking your phone. Unfortunately, they have a little too much power in the decision. But there's another option. There is always another option.
Use an unlocking service
There are companies who buy network unlock codes in bulk from carriers, then resell them to the public. And while legally this is a grey area that changes from year to year (based on the whims of people like the Librarian of Congress), we can let them worry about the law and we just use the service they provide.
If your carrier won't help you can use a third-party.
You unlock your phone mostly the same way you would if you were doing it through your carrier. You contact an unlocking service and pay their fee (usually between $20 and $30) and they send you a code. When you insert your new SIM card, you use this code to unlock the SIM.
Again, you may have to enter some network settings, and the people who sold you your new SIM card can walk you through this. Or you can visit the forums and get the same information from people just like you who have already done it.
There are plenty of other services out there, and I suggest you use the forums and see what your fellow users have to say about them.
Now you're armed with some knowledge. You know the benefits of having an unlocked phone, and know how to get your phone unlocked if you need to. Share this with the folks you know who need to know it and stop letting the people who put their name on the back of your new phone tell you where you can use it.
Updated January 2018: This post was updated with current information about unlocking and how to go about doing it.