Simple phones that work on O2

2G: On. O2 plans to start switching off 2G in summer 2029.
4G calling for phones they didn't sell: Approved phones only. O2 says 4G calling works on "most O2-compatible devices" and needs a recent SIM. It doesn't mention feature phones.

Phone	Verdict	Why
HMD 105 4G	Works (on 2G)	Calls and texts work on O2 using 2G. They'll stop working when 2G is switched off unless 4G calling gets enabled.
HMD 110 4G	Works (on 2G)	Calls and texts work on O2 using 2G. They'll stop working when 2G is switched off unless 4G calling gets enabled.
HMD 2660 Flip 4G	Works (on 2G)	Calls and texts work on O2 using 2G. They'll stop working when 2G is switched off unless 4G calling gets enabled.
HMD 3210 4G	Works (on 2G)	Calls and texts work on O2 using 2G. They'll stop working when 2G is switched off unless 4G calling gets enabled.
HMD Fusion X1	Works (on 2G)	Calls and texts work on O2 using 2G. They'll stop working when 2G is switched off unless 4G calling gets enabled.
Mudita Kompakt	Works (on 2G)	Calls and texts work on O2 using 2G. They'll stop working when 2G is switched off unless 4G calling gets enabled.
Alcatel 3080G	Works (on 2G)	Calls and texts work on O2 using 2G. They'll stop working when 2G is switched off unless 4G calling gets enabled.
Punkt MP02	Works (on 2G)	Calls and texts work on O2 using 2G. They'll stop working when 2G is switched off unless 4G calling gets enabled.

Sources