There used to be such a thing, called a mint card I think. Charges were very
high though.

The only way it can be done is with a foreign SIM card with roaming
agreements with all Brittish nets.

But then you'll have to pay for incoming calls.

http://osbymikro.tk =================== phone pages
http://2sms.tk ======================== free sms
info@osbymikro.tk.remove_th.is ======== mail address

you need a sim copier device,it's very common in Turkey
nowadays.up to 8 different network sim card can be copied into one silver
smart card.

Thanks so far for all your advice.

One thing I didn't mention is that this isn't for voice calls - it's for
GPRS data transfer.

Also, it needs to be legit!  It's for a work project.  We've got our system
working sending data over each GPRS network individually, with the
appropriate modem configuration, but to switch to another network means
manually replacing the sim card, then changing the modem config.

This multi-network sim card I need needs to be able to be changed from one
network to another just by sending new config to the modem.

Eno

Yes. I have one. It's my French mobile.

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8 in 1 SIM
There is a product, available thru Online Etailers , some call it the 8 in 1
card. Its going to run you about $80-$100 USD to get the Blank Card. The
Reader/Writer Hardware and the Software. It receives very mixed reviews.Also
keep in mind, One Line at a time. It will install anew MENU on your phone
through SIM TOOL KIT. You turn c phone on enter a security code and select
tyhe number you want.
http://phonetoys.com.hk/wholesale/product_info.php/products_id/264

http://ucables.com/ref/SIMCARD8

I believe that a Jersey telecom will roam all 4 networks in the UK.

I was talking to them about telematics and they said they have a solution
though their agreements with the operators.

You can effectively use it as a GPRS channel anywhere in the UK, so as
signal is lost in one area for a network, you move to the next network etc.
Great for the highlands.

WRT the charging structure, I don't know. You would have to speak to them.

Hope that helps