O2's flexible plans let you change your data allowance each month

When signing a new mobile contract, you typically pick a monthly plan with strict call, text and data allowances. You're then tied to that decision as you see out the contract for the next two years, before starting the process all over again. With O2's new flexible tariffs launching tomorrow, however, you can jump between different plans each month, shrinking or growing your data allowance, and thus how much you pay that month, too.

You might want to make sure you have a larger data allowance in a month you anticipate uploading a lot of pictures, for example, or a more frugal plan if you're going on holiday and expect to spend more time looking at vistas instead of your phone. Subscribers can select a different plan once per calendar month, in-store, over the phone or via O2's app, with the new allowance kicking in next billing cycle. You only need to worry about how much data you'll need, really, since every relevant O2 plan includes unlimited calls and texts as standard.

This flexibility is available to new and existing O2 Refresh customers from tomorrow, but there is, of course, a catch. You need to have a relatively new phone supplied by O2 to qualify: Either of the new iPhones or the soon-to-be-released iPhone X, a Galaxy S8, S8 Plus or Note 8, or a OnePlus 5. O2 is in a slightly unique position to offer shapeshifting plans because of the way it structures monthly payments.

O2 Refresh portions your monthly bill into two parts. One is the charge for paying off your handset, and the other is the charge for your mobile service (calls, text and data). This means O2 can make the latter charge flexible without changing the amount you pay for the actual phone each month. The carrier is understandably keen to promote its transparent pricing at the moment, after Citizens Advice recently reminded people they are at risk of overpaying if they don't upgrade immediately when their current contract expires.

Most mobile providers bundle your phone and service bill together, you see, so if you continue to pay the same amount after your contract is up, you're effectively giving the carrier free money since you've already paid off the cost of your handset. But O2 isn't the only provider that splits bills in this way. Sky Mobile does the same thing, for example, and offers a similar style of flexible plan you can change each month -- incidentally, the MVNO piggybacks on O2's network. There is a benefit to Sky's way of doing things, though, in that any data you don't use that month goes into a pot you can tap if you exhaust your allowance in any following month.

Rollover data is not a feature of O2's flexible plans, however. If you think it's going to be a particularly busy month and you want a 50GB cap to tide you over, but you only end up using 10GB of that. Well, the other 40GB you paid for is lost forever.