SIM card not working on 2G

[oisgreatreally]

I have an old 2G only device (it's an alarm signaller) which works fine.

I recenty tried a newly supplied o2 contract SIM in it and it does not work.

Older o2 contract & PAYG SIMs seem to work fine in it still.

Is there something different about new o2 SIMs that would stop them working in an older device?

Thanks.

[MI5]

@oisgreatreally 

From memory, O2 replaced all the old 2g sims last year as network changes came into effect which would render them useless. 

New sims should, however continue to work OK.

Are you sure you got the correct type of sim for your account?

[oisgreatreally]

Can you clarify, what kind of SIM should I be asking for please?

[MI5]

@oisgreatreally 

Tariff specific, so either PAYG or monthly contract.

[oisgreatreally]

It is a contract SIM, sorry I am not really sure what you are asking me to check here.

[MI5]

@oisgreatreally 

That you have a contract sim for a contract tariff ?

If you have, it should work so did you activate it when you got it?

[oisgreatreally]

The SIM works ok in another device so I don't think that's the issue.

[MI5]

Might be worth popping into your local O2 store with your device and sim @oisgreatreally 

I doubt this can be diagnosed remotely.

Store locator | O2 UK | Find Your Nearest O2 Store

[oisgreatreally]

According to our local o2 shop, contract sims are now only 45/5G.

PAYG SIMs still have full 3G capability so because this is closer to 2G they would probably still work for now.

However, O2 customer services reckon that a 5G SIM will work in any device including 2G.

Do any of these sound right?

[MI5]

@oisgreatreally 

the shop is talking rubbish.