Harry asked about our roaming partners on Monzo’s forum and @ben seconded this last year, so… we don’t have that list (it’s ever-changing, and the agreements are managed by our host network), but I thought I’d shed some light on how the whole roaming negotiation thing works.
Twice a year, operators meet at the Wholesale Agreements and Solutions (WAS) conference* to discuss exactly this. In speed dating-like sessions, they agree on what they’ll charge each other for roaming – data access, calls to mobiles, calls to landlines, text messages, etc., etc. Having said that, I think the bigger deals are done directly.
And when you roam…
Your home network prioritises which operator you connect with, and depends on a few things – cost, connection quality, who sends them traffic, 4G availability, whether they’re part of the same group (e.g. Vodafone, Three…), etc.
You can manually override this (depending on the handset), but only to the operators your network has a roaming agreement with. Your SIM card stores a list of those operators (called a PLMN list; we don’t have access to it) which they can update remotely, and if you go into the network settings on your phone and disable automatic selection you can see them.
*The next one’s in April with 280+ operators; we’re still waiting for an invite…