(Pocket-lint) - TalkTalk has launched what seems to be an unbelievable offer for a 3G SIM. For just £12 a month - and on a rolling 30-day contract to boot - you get unlimited talktime, texts and 3G data, a deal that the company claims is the cheapest on the market.

There are other dead cheap SIM only plans around, especially if you aren't bothered about leaping to 4G yet, but it must be said that £12 seems to be a cracking deal when there's no monthly data limit.

However, there are caveats - massive ones.

To get the £12 a month deal new customers will need to also take TalkTalk's broadband service. Existing customers can also get the service, but if either cancels their broadband account, the price of the SIM tariff doubles, to £24 a month.

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In addition, you can't tether using the SIM, nor can you use it for anything other than a smartphone - although this latter restriction is not uncommon.

That does water down the deal a tad, but while Virgin Media is the only rival offering a similar 3G package, it has recently capped its data limits for new customers and you'll have to pay £17 a month for 1GB of data if you don't take its broadband, TV or a home phone too.

You can find out more about the TalkTalk All-in SIM here.

Writing by Rik Henderson.