..
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> Vodafone Multisim will do this although I believe it stops data working, and
> Orange offer a similar service for their business customers.

Vodafone do indeed do multi sim, however, it's very expensive,
and all you can do is calls and sms. No grps, no data, no mms
etc etc.

The multi sim price plans are multi sim specific, they don't
come with any x-net minutes, and x-net calls are about 50ppm
on the multi sim price plan, I think it's £30 a month for
sim 1 with 200 minutes off peak to landlines and vodafones,
and then each other sim is a further £15 a month plus vat.