United Mobile+ (new service with +44 Jersey number)

[inquisitor]

Originally Posted by DRNewcomb

It would be virtually impossible for them to distinguish US cellular phones from landlines.

There's no need to distinguish in the case of the US, as mobile and landline calls are terminated at the same rate.
United Mobile don't charge higher rates for mobile calls in other countries because they think it's funny, but because termination rates (the price the receiving operator demands) of those networks are higher.
Whereas in the US the called mobile subscribers pay the extra cost for the transfer to their mobile phone, in the rest of the world callers pay the price for the whole connection.
The high costs for American mobile subscribers, who also pay for incoming calls, are a main reason why mobile phone penetration is relatively low in the US.
The low penetration again is the reason, why prices for mobile telephony are higher - American operators must finance their large networks by a relatively small number of subscibers and further there are quite few players in the GSM market, so competition is limited. Beyond that the geographical expanse of the US requires more basestations and so results in more costs then in other places, where population density is higher. Also in European countries the investment in GSM networks has already amortized years ago, while US operators, of whom some migrated to GSM recently, still wait for the return.
All this makes mobile telephony more expensive in the US than in other places.