Other than the fact that your SIM is TMobile and other SIMs are O2, is everything else identical (i.e. same Blackberry model, same or similar software version)? I have seen VPN support vary with handheld OS version (normally newer ones support more features than older ones though, of course).
You may have already done this, but to rule out the Cisco client side of things I'd try using a known-not-to-cause a problem colleague's Blackberry and have them use yours as a test. That should pin it down to either the device+network or the client PC. If your devices are unlocked you could try swapping SIMs as well.
Unfortunately I don't think that I've ever used a Cisco VPN over a Blackberry connection (with either O2 or TMobile) so can't help answer the direct question.
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