I've noticed quite a few places advertising free or cheap sim-only PAYG deals.

I was thinking of getting one, bunging it in an old phone and leaving that phone in the missus' car, with the battery out, to ensure she has a phone in case of emergencies.

FWIW, I'm considering this because we both have smartphones and the battery usage is such that you can't always rely on them still being useful after a couple of days without charge.

My own phone has a PAYG O2 sim and there's been times when I haven't put credit in it for over a year and it's still working so I know O2 sims aren't limited in any way.

I was just wondering if there ARE any PAYG sim-only deals that require you to top the phone up every month or use the phone every month so I can steer clear of them?

All will at some point get dis-connected if you don't use them though this may be up to a couple of years down the line. There is nothing to prevent you not topping up each month just incentives if you do, such as 100 free texts.

Normally it's 6 months of inactivity before a pay as you go sim will deactivate. You simply need to make a chargeable call or send a chargeable SMS to prevent this. Credit will last indefinitely normally as long as you do this

I have a Orange PAY SIM that I use in my back up phone. I have only put £20 on it in around 4 years and had no problems.

Texts these days are pretty cheap e.g. 4p so send a text once every 6 months to keep the sim active.
You can always find the inactivity period clause in the T&Cs of the sim or just call up the network and ask them what it is.

Thanks for that.

I'm sure I can remeber to use it once every 6 months. I was a bit worried that some might require use every month or summat.
That's be a bit of a PITA.