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[inquisitor]

Originally Posted by ronwi

Lycamobile US uses Tmobile only as well, so I suspect they have some special deal with Tmobile. The fact that they have free incoming calls in the US without a US IMSI (I think they are the only mobile operator that does this) suggests a highly negotiated deal with Tmobile.

That's interesting because according to p.2 of this document Dutch Vodafone customers are charged higher rates when roaming on AT&T than on other US networks. Hence you should assume that Vodafone NL's wholesale purchase price for airtime on AT&T is higher and so other roaming networks should be preferred. But there are many possible reasons why toggle does not roam on T-Mobile - one of them could be lacking support of the required CAMEL phase that enables realtime billing, a basic requirement for prepaid tariffs.
Anyway, I expect toggle to start assigning American IMSIs from Lycamobile sooner or later and then customers will be locked to T-Mobile.