The Quiet Cost of a Spare Phone: A 2026 Low-Use SIM Audit

There’s a particular kind of UK phone that never leaves the house. It sits in a kitchen drawer, charged to 43%, waiting for a delivery driver, a power cut or a grandparent who prefers a landline but needs a mobile for the odd taxi. The SIM inside looked like a bargain: £5, maybe £1 on a promotion. The trouble is that a cheap SIM is not the same as a cheap habit.

For occasional UK callers, the headline price is almost irrelevant. What matters is how often you must top up, whether that credit expires, what happens when you call outside a bundle, and how long the number survives if you forget about it. Run those numbers and a so-called cheap PAYG SIM can turn into a £60 annual subscription you never meant to buy.

The headline price is just the entry fee

A starter SIM is often sold like a packet of biscuits. It is small, cheap and easy to pick up in a supermarket. But the shop price only buys you access to the network. The real cost begins with the minimum top-up.

Many PAYG plans require a top-up every 30, 60 or 90 days to keep the line active. A £10 top-up every three months is £40 a year before you have made a single call. Some plans let credit roll over if you top up often enough. Others treat unused credit like milk: fine for a while, then past its date. The small print usually calls this credit expiry, validity or a use-by period. It rarely appears in the bold type on the shelf.

If the phone is for emergencies, you might assume you can leave it alone for months. That is exactly when expiry clauses do their best work. A phone with no active credit may still be able to call 999, but it cannot receive the callback from a carer, a GP surgery or a delivery driver. The emergency-only argument is sound for outbound 999 calls, but it is a poor fit for everyday usefulness.

Where a low-use SIM quietly charges you

There are five usual suspects. None is scandalous on its own. Together, they explain why a drawer phone can cost more than a cheap monthly SIM.

1. Keep-alive top-ups

This is the big one. If the plan demands a minimum top-up every 90 days, you are paying £40 a year for the privilege of doing nothing. Some networks allow smaller top-ups or longer gaps. Others are stricter. Check the interval, not just the amount.

2. Credit expiry

Credit that expires is worse than a high price, because you cannot see it leave. Look for phrases like ‘credit valid for 90 days’ or ‘top up every 180 days to keep your number’. If the credit expires, your annual spend is not the top-up total. It is the top-up total minus whatever you actually used, plus the cost of replacing what vanished.

3. Out-of-bundle rates

A bundle that includes 500 minutes is useless if you make 12 minutes of calls a month and the bundle only renews when you top up £10. Outside the bundle, calls can be charged per minute at rates that look like a premium phone number. Data is similar. A quick map check or a photo sent to a relative can trigger a per-MB charge that feels almost old-fashioned. For a low-use line, per-minute and per-MB rates matter more than the bundle headline.

4. Auto-renewing bundles

Some PAYG bundles renew automatically from your credit. That is convenient until you forget the phone exists. A £5 bundle every month is £60 a year, and you may not notice until the credit runs out and the line goes dormant. Ask whether the bundle auto-renews and how to stop it.

5. Inactivity and number recycling

A UK number is not yours forever. Networks can reclaim numbers after long periods of inactivity. The exact window varies, and it is usually buried in the terms. If the spare phone is also the number your relative gives to the pharmacy, losing it is more than an admin annoyance. It is a small security risk, because a recycled number can end up with someone else.

A worked example: the £5 SIM that becomes £65

Here is an illustrative annual audit. Your own plan will differ, but the shape is familiar.

Cost	Typical small print	Annual effect
Starter SIM	£5 one-off	£5
Keep-alive top-ups	£10 every 90 days	£40
Occasional calls	10 minutes a month at 25p/min outside bundle	£30
One data pass	1GB for a delivery app week	£5
Total		£80

That is not a disaster. It is also not a £5 SIM. If you trim the calls and skip the data pass, you are nearer £50 to £65 a year. The point is that the cheap headline has become a modest annual habit. A low-cost monthly SIM with a spend cap might cost similar money and be far easier to understand.

The elderly-relative test

A spare phone for an elderly parent or grandparent deserves a stricter audit. The person using it may not check credit, read expiry texts or notice that a bundle has stopped renewing. A line that goes dead is not just inconvenient. It can cut off the number the family uses for check-in calls.

Look for a plan with a simple monthly cost, inclusive calls, a spend cap and no surprise out-of-bundle data. If the user only needs a phone for occasional calls, a cheap monthly SIM can beat PAYG on predictability. If they need a true emergency handset, keep it simple and test it once a month. A phone that cannot receive calls is not a safety line; it is a plastic brick with a lock screen.

A pre-purchase checklist

Before you buy or keep a spare SIM, ask these questions:

• What is the minimum top-up, and how often must I pay it?
• Does unused credit expire? If so, when?
• What are the per-minute call and per-MB data rates outside a bundle?
• Does the bundle auto-renew, and how do I turn that off?
• How long can the line be inactive before the number is recycled?
• Can I set a spend cap or block chargeable extras?
• Is Wi-Fi calling included? It matters if the signal indoors is poor.
• What does a replacement SIM or eSIM cost if the phone is lost?

The drawer phone deserves a real budget line

A spare phone is a useful thing. It can keep a delivery driver connected, give a relative a simple way to call, or sit ready for a broadband outage. But ‘cheap PAYG’ is a pricing story, not a promise. The annual cost is the sum of top-ups, expiry, out-of-bundle rates and the value of a number you do not want to lose.

Before you add another SIM to the drawer, spend ten minutes with the terms page. If the numbers look fuzzy, pick a plan with a clearer monthly cost. A £5 SIM is only a bargain when the year behind it stays small.