I would suggest asking around your friends & family for anyone that may have an older phone sitting unloved in a draw. The reason I suggest this is that both Android & Apple have recently updated their operating systems & there seem to be issues with both. An older phone, especially a tatty one, may also be less likely to disappear!
Android would probably be best as Apple phones require an Apple ID, which in turn requires a payment method to be assigned.
So all you need is a phone running Android 8 or above with NFC capability for local scanning & bluetooth for the desired CGM updates. Here's the compatibility list for the latest app from Abbott.
Samsung
Galaxy A7, A20, A20e, A21, A22, A23, A30, A32 5G, A40, A41, A52, A52
5G, A70, A71, A71 5G, Feel2, Note8, Note9, Note10+, Note10 Lite, Note20 Ultra
5G, S7, S7 Edge, S8, S8+, S9, S9+, S10, S10 5G, S10e, S10+, S20, S20 FE 5G, S21
5G, S21 5G Olympic Games Edition, S21+ 5G, S22, Z Flip3 5G, Z Fold3 5G
Asus
Zenfone 8
Fujitsu
Arrows 5G, U
Google
Pixel, Pixel 2, Pixel 2 XL, Pixel 3, Pixel 3 XL, Pixel 4, Pixel 4 XL, Pixel 4a
5G, Pixel 5, Pixel 6, Pixel 6 Pro
HTC
Desire 22 Pro
Huawei
P20 Pro
LG
style3
Motorola
moto g 51 5G
OPPO
Reno A, Reno3 A, Reno5 A
Sharp
AQUOS R3, R6, sense3, sense3 plus, sense3 plus
Sony
Xperia 1 II, 5, 8, Ace II
Xiaomi Redmi Note 9T