Unable to message internationally on prepaid plan

I have a prepaid mobile plan which is supposed to include unlimited international messaging but at the moment I am unable to text internationally, even when ensuring I use the correct country codes. I am an exchange student from the UK and I signed up to this plan when I moved to America so I could stay in touch with friends and family at home. Is there any way I can unblock international messaging or otherwise rectify this issue?

Thank you.