Start by rebooting your phone. Then make sure your SIM card is working< try it in another phone? if that doesn't work then I would say message support.
Are you trying to port your number over from another carrier? If so the number has to still be active at the time. if that didn't work perhaps you missed the authorization text to allow the swap? perhaps call the other company and authorize the port....
you are able to buy international sim options for travel. The other option I have done is using apps for texting instead of relying on my phone service for SMS. try text me app it's free and worked for me no problem ! Hope this helps.
