CPI Plus 3.9%: The SIM Contract Clause That Keeps on Charging
Photo: Row17, CC BY-SA 2.0, via Wikimedia Commons
SIM-only deals are sold like a monthly subscription: one neat price, no phone, no fuss. Then the small print arrives with a calculator. In 2026, plenty of UK SIM-only contracts still include an annual price-rise clause tied to inflation plus a fixed percentage. That is not automatically illegal. It is not automatically fair, either. The trick is knowing what you are signing before the first bill lands.
What CPI plus 3.9% actually means
CPI is the Consumer Prices Index, a common measure of inflation. RPI is the older Retail Prices Index, which usually runs higher. A clause that says CPI plus 3.9% means your monthly bill goes up by the inflation figure plus another 3.9 percentage points. If CPI is 3.5%, the rise is 7.4%. On a £20 plan, that is £21.48 a month. Next year, the same formula applies to the new amount, so it compounds. RPI plus 3.9% can sting more. The plus bit is not a rounding error; it is the margin.
Since January 2025, Ofcom has required providers to show future price rises in pounds and pence at the point of sale. That was a genuine improvement, but it did not ban the clauses. It just made them harder to hide. You still need to check the contract summary and ask what the rise means for your plan.
Which UK networks include them?
The big four have used these clauses on many pay-monthly plans, and 2026 is no clean break. EE, Vodafone and Three have commonly used CPI plus 3.9%. O2 has used RPI plus 3.9% on many plans. Smaller brands vary wildly. Some mirror their host network's terms; others sell 30-day rolling plans with no annual mid-contract rise. giffgaff and Smarty generally avoid annual contract rises because they sell flexible monthly plans. Lebara and Lyca often sell 30-day plans too. Tesco Mobile has marketed no mid-contract price rises on some deals, but check the specific plan.
That is why the brand name is not enough. A virtual network can be a safe haven or a copycat. The clause lives in the contract, not the logo.
The legal bit: when a rise is allowed
A price-rise clause can be legal if it is clear, incorporated into the contract and fair under the Consumer Rights Act 2015. If it is buried, vague or applied in a way the contract does not support, it is open to challenge. If a provider raises your bill by more than the stated formula, that is a breach of the terms. If they change the contract in a way that causes material detriment and you did not agree to it, you may have a right to exit without penalty. If the rise is exactly what the contract said would happen, you usually do not have an automatic penalty-free exit just because you no longer like the number.
How to spot the trap before you sign
Look for phrases like annual price adjustment, inflation-linked rise, RPI, CPI or 'we may increase your monthly charge'. Check the contract summary, not just the headline deal page. Ask customer service a very specific question: what will I pay after the first annual rise, in pounds and pence? Get the answer in writing. Check whether the rise applies to the whole bill or only the airtime part. Check whether a promotional discount expires at the same time. Then do the boring maths: compare the total over 24 months, not the first month. A £12 plan that rises twice can cost more than a £14 flat plan.
If a rise lands on your bill
Read the notice and calculate the increase. If it matches the clause, you have a choice: accept it, switch at the end of the contract or ask for a retention deal. If it does not match, complain to the provider first. Keep screenshots of the original offer and the contract summary. If you believe the change is to your material detriment, ask for a penalty-free exit in writing. If the provider says no, you can escalate to its alternative dispute resolution scheme after eight weeks or once it has issued a deadlock letter. Do not simply cancel the direct debit; that can damage your credit file and will not end the contract cleanly.
The bottom line
Inflation-linked price rises are not evil in themselves. Costs do go up. But CPI plus 3.9% is a pricing strategy with a friendly name, and it belongs in your decision, not in a footnote. If you want certainty, look for a flat-price plan or a rolling 30-day deal. If you accept a rise clause, accept it with your eyes open and a calculator nearby.
