Well OK then I don't really care about 7624 or 7924.
What's about the roaming agreements, you seem to know them so do you also know which one have more agreements ? My guess is UM as they are the oldest but what's about GlobalSim for example ? I'm not asking for a precise comparative but let's limit it to Europe.

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...