Answer: when T-Mobile uses the word "UMA" as its network ID for all roaming customers, instead of "T-Mobile US".
No idea why this happens - initially on an unlocked Nokia E71 with a T-Mobile SIM (I thought it maybe had a "hidden" UMA stack and auto-logged on to the SFO Airport network, until I switched off the WiFi). Then I got the same tag on my non-WiFi SonyEricsson, and realised the answer must be simpler.
I'm pretty sure it's always just said "T-Mobile US" when I've been here in the past.
Speaking Engagements & Private Workshops - Get Dean Bubley to present or chair your event
Need an experienced, provocative & influential telecoms keynote speaker, moderator/chair or workshop facilitator?
To see recent presentations, and discuss Dean Bubley's appearance at a specific event, click here