You can find the roaming agreements on the "original" operators' sites:
-
FL1 for United Mobile
-
Manx for IoM based sims
-
EMT for Travelsim
-
Siminn for 09 Mobile
Or you can look on GSMworld at the
roaming page, looking for the above mentioned operators.
Anyway SMS interconnection will always have some problems, and considering that SMS usually are quite expensive too while roaming, I think it's not a great idea to use them if you want to be sure to communicate with someone else...