Sim only - occasional, light user - cheapest

Speedsie

Hello,

I'm looking for the best sim only plan for an elderly, very occasional user. Calls only will be used.
It is an emergency backup for a landline, so need to have a long expiry date on phone numbers - ie a live call made every 6 months or so.

She used to have an eir sim, but didn't make a call for about 8 months, and the number got recycled. (I know, I need to set up a diary reminder!!!)

Before I head to Carphone Warehouse at lunch, does anyone have any thoughts?

She would mainly be calling landlines and a variety of different mobile providers.

Thanks in advance!