While you can add most numbers worldwide to your account, which numbers you may add depends on both where the number is located, and which plan you have on your account. You can check any number, and which plans it is available under here.
There are some countries, regions, carriers, and even specific numbers where we specifically block and prevent numbers from being added and verified to any TrueCNAM account. These blocked areas and numbers are typically blocked because they are in high cost areas, or because of high levels of fraud.
Finally, of course, you must "own" the phone number you are trying to add, as it will be verified by calling or texting you before it can be added.