Really? I was pretty active on mixi back in the day, and I'm almost 100% positive it didn't require a cell phone email address at all, let alone specifically one in any particular region. (My login ID was my PC email account.)
(edit: Double-checked this on Wikipedia.)
Apparently it originally required a PC email address to register, then from 12/2006 began allowing cell phone email addresses, then from 3/2007 began requiring a cell phone address as backup confirmation for people who registered with free email provider accounts. There was no such requirement for those registering with other email addresses, such as ones linked to an internet provider. (It goes on to mention requiring a cell phone address down the line, but as I recall, this was long after the service had fallen from popularity.)
I can't remember whether or not there was a (limited) English-language interface, but it doesn't really matter, as 99.9999999% of the users were Japanese native speakers interacting entirely in Japanese, anonymously and centered around communities based upon their interests (musical artists and bands, authors, sports, hobbies, etc.) The users would often get together for オフ会 (in-person gatherings of online friends).
I have a lot of fond memories of mixi, as I met some really cool people through it when I first came to Tokyo, some of whom ended up close real-life friends. (My now-wife is actually the friend of a friend I originally met through mixi.)
Anyway, there's really no point in joining mixi anymore, as (1) barely anyone is active on it these days (the membership was already dwindling in the latter days I was using it, and I don't think I've logged in in over five years), and (2) the small number of people still using it are most likely just doing so to interact with people they met a long time ago.
Even in its heyday, again, it wasn't an "instant messaging" service but rather a community and diary-based site designed to bring together people with the same hobbies. People rarely used their real names or posted photos, they talked about their interests rather than humblebragging about their lives and accomplishments, and so on and so forth. There isn't really anything like it today (but now I'm just getting into nostalgic old person mode.)