BT, much like the Royal Mail, have to offer a "Universal Service", Mercury/TNT/citilink/talktalk have no such obligation.
BT are obliged to route calls to any accessible line, and to do so within a scale of pricing that is not impaired by political or geographical constraints.
This is why "national" Lands End to John O Groats costs the same as "national" Bristol to Reading. BT loses (or makes less) money routing the extremely long distance calls and cleans up on the short range calls. Other providers can bill based on the actual material cost of routing the call, and can decline to route calls that will lose them money, thereby keeping their rates lower.
T-Mobile have a very strong case in this instance. Since arguably they are losing money by paying the wrong termination rate. (Which in itself is arguably mis-sale of services.) And also have no inclination or oblgation to sacrifice their profits to help an otherwise unsustainable start up. Claiming that T-Mobile are a "monopoly" won't help either.
Slashdot readers, with one hand as ever firmly in their crotch are of course hailing this as some great victory for David over Goliath... Once again demonstrating what happens when Americans try to pass judgement on situations they don't understand... you'd have thought they'd have learned by now...