In order to send SMS, or MMS, to an international network, or to receive them from a foreign network, the 2 networks in question need to have mutual agreements.
So if you have problems sending or receiving messages to / from abroad, it is helpful to be able to check whether such agreements are in place to begin with.
A helpful tool for checking can be found
here..
I am told that this site is sometimes not up to date, but search results on it may be helpful in identifying the cause of the problem you are experiencing.
If anyone knows of other resources for checking such information, please post them below here.
Thanks.