Millions of mobile phone users could save a small fortune by making a quick and simple check.
Martin Lewis is urging those who are out of contract, an estimated nine million people, to see if they could make savings.
The Money Saving Expert founder says the cost of a sim card has plummeted over the last year.
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One Money Saving Expert customer, David, emailed in to say he had managed to save £170 per year by using its Cheapest Sim tool.
David said: "Bagged myself a £7/mth Sim deal - a massive saving on the £21.50/mth I was paying. Many thanks."
Martin said if you want to stick with your network's signal, you can select this in the tool's options.
He also suggests around 3GB of data per month as an average, unless you're a "gamer or video streamer".
Money Saving Expert also has a cheap mobile finder for those in need of a new handset.
Deals, Martin said, are really cheap right now.
One example is TalkMobile (which has Vodafone's signal) which is giving new customers 10GB of data, unlimited minutes and texts for £7 a month.
Customers can also claim a free £30 Amazon voucher.
With this factored in the deal is equivalent to £4.50 a month over the one year contract - considerably less than many of us are paying.
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