The quick answer

• iPhone 6 / 6 Plus (2014) — Still makes calls and texts on the major US networks. But it has been frozen on iOS 12.5.7 since January 2023, with no security updates since. Workable as a backup, not sensible as your only phone.
• iPhone 6s / 6s Plus (2015) — The meaningfully better of the two. It reached iOS 15, so it still runs a large share of current apps. Genuinely usable as a light daily phone or a second line.
• Neither has 5G, neither has eSIM, and neither supports T-Mobile's Band 71 — so coverage will be worse than a modern phone on the same plan and the same tower.
• Neither died from the 3G shutdown. Both support LTE and VoLTE. Software is the limiting factor, not the network.

Why the 3G shutdown never touched these phones

When carriers retired their 2G and 3G networks, the phones that stopped working were the ones that depended on those networks to place a call. Voice traffic used to run on the older network even when data ran on LTE — so a phone without Voice over LTE lost the ability to dial, even though its data still worked.

The iPhone 6 was the first iPhone generation to support VoLTE and Wi-Fi Calling, and the 6s carried both forward. That is the specific reason these two survived a cull that took out a lot of Android handsets from the same era. If your iPhone 6 was active before the shutdowns and kept working through them, this is why.

The real cutoff is iOS, and the two models are years apart

This is where the 6 and the 6s stop being interchangeable, and it is the single most useful thing to know before you decide whether to keep one.

• iPhone 6 and 6 Plus — final release was iOS 12.5.7, issued in January 2023. That was a security patch, and it was the last one. Everything since has shipped without them.
• iPhone 6s and 6s Plus — reached iOS 15 and received patches in the 15.8 range well after that. Three major versions further along, which in app-compatibility terms is an enormous gap.

You cannot update past those ceilings. Apple ties iOS versions to hardware, and no setting, reset, or workaround moves a phone beyond its final supported release. When you see an iPhone 6 offered as "upgradeable," it is not.

What actually breaks first

Not the phone. Apps. Every app sets a minimum iOS version, and developers raise it over time. When an app's minimum passes your ceiling, the App Store quietly serves you the last version that still ran — which then never updates again. Nothing announces this. The app simply stops changing, and eventually stops working when the service behind it moves on.

Three consequences worth taking seriously:

• Banking and authentication — these are usually the first to drop old iOS versions, and they are the worst ones to lose. If your phone is also your 2FA device, a frozen authenticator app is a real risk, not an inconvenience.
• Messaging — cross-platform messaging apps have been steadily raising their floors past iOS 12, which is the iPhone 6's ceiling. The 6s has more headroom here.
• The browser — on iOS, Safari updates with the operating system rather than separately. A frozen iOS means a frozen browser engine, and over time more sites break outright rather than merely looking dated.

Coverage: the bands these phones are missing

Two phones on the same plan, standing in the same spot, do not necessarily get the same signal. Coverage depends on which frequency bands the handset supports, and an older phone simply cannot see the newer ones.

The important gap is Band 71, T-Mobile's 600 MHz spectrum. Low-frequency bands travel further and penetrate buildings better, which is exactly why T-Mobile uses it for rural and indoor coverage. Neither the iPhone 6 nor the 6s supports it — that band arrived in later iPhone generations. On T-Mobile or a T-Mobile-based MVNO, these phones will show weaker signal in precisely the places where coverage is marginal.

Wi-Fi Calling offsets some of this indoors, and both models support it. It does nothing for you in the car or out of range of a network you trust.

No eSIM — and one consequence people do not expect

eSIM arrived on iPhone with the XS and XR generation in 2018. The 6 and 6s are physical-SIM only, and no adapter or update changes that. That rules out any plan that activates by eSIM alone, and it has a second effect that catches people out.

Nearly every free carrier trial now runs on eSIM. That is the whole design: the trial line sits alongside your existing one so you can test a network without giving up your number. On a phone with a single physical SIM slot and no eSIM, that is not possible — you would have to pull your current SIM and lose service while you test.

Keeping an existing line versus activating a new one

These are not the same task, and the difference surprises people. A line that already works tends to keep working — carriers rarely go back and disconnect a functioning device. Activating that same phone fresh, on a new account or a new carrier, means passing a device-compatibility check first, and those checks are stricter than the network's actual requirements.

So an iPhone 6 that has been running fine on the same account for years may still be refused when you try to move it to a cheaper plan. Check the IMEI against the carrier you want before you cancel anything — every major carrier and most MVNOs publish a compatibility checker, and it takes a minute.

Battery and parts, ten years on

A 2014 or 2015 battery has been through its full cycle life several times over. Expect a phone that reads 40 percent and dies, or shuts down in the cold. iOS will also throttle peak performance on a degraded battery to prevent those shutdowns, which is why an old iPhone feels slow in a way that a factory reset does not fix.

Apple has moved the iPhone 6 to obsolete status, which means official parts and service are no longer available for it. Third-party battery replacement is inexpensive and still the single best way to make one of these usable again — but it is worth doing only if the phone has a job to do that does not involve the modern internet.

Who these phones still genuinely work for

There is a real answer here that is not just "upgrade." Both models remain useful in roles that do not depend on current apps:

• A glovebox or emergency phone — VoLTE means it can still dial, including emergency calls, on an active line.
• A first phone for a child, on a cheap talk-and-text plan, where limited app support is arguably the point.
• A dedicated music player, alarm clock, or offline GPS unit over Wi-Fi, with no SIM at all.
• A second line for work calls, where the only app that matters is the dialer.

What they are not suited for: banking, two-factor authentication, anything holding sensitive data, or being the phone you rely on when something goes wrong. An iPhone 6 has gone three and a half years without a security patch. Treat it accordingly.

If you are replacing it

Two things are worth knowing before you shop. First, you do not have to buy a phone from a carrier — every major network and nearly every MVNO supports bringing your own unlocked device, and that is almost always cheaper than a financed handset. Second, if your last plan was chosen a decade ago, it is very likely wrong for you now: data pricing has changed enormously, and most people carrying an old iPhone are on far more data than they use.