International calling isn't part of your plan
No Flex Mobile plan includes calling to other countries. That's true of Flex Standard, Flex Unlimited and Flex Unlimited Plus alike, so dialing an international number directly won't work.
Roaming and international calling are different things. Roaming is using your phone while you're in Canada or Mexico, which the unlimited plans cover. International calling is dialing another country from here, which no plan covers.
The free alternative
Calls made through WhatsApp, FaceTime, Messenger or similar apps travel over your data or wifi connection rather than the phone network, so no international calling rate applies at all. If the person you're calling uses the same app, this is usually the simplest option.
Texting abroad
A text to an international number is charged differently from a text within the US. Messaging apps avoid that entirely.
Before you make a habit of it
Check your usage after the first month. If you call one country regularly, it's worth asking Flex Mobile support whether there's a better option for you.
If the call won't connect
Check the country code, and that you've dropped the leading zero from the local number. If it still fails, contact Flex Mobile support with the number you're trying to reach.