I'm looking for a cheap mobile phone to use with an iSim prepaid sim
card (ie Optus GSM network) that can be configured to only dial numbers
that are already stored on the sim card, and nothing else.

I have an old Nokia 5110 that suggests it has this mode of operation,
but the way it's implemented means that you have to know and type in the
number, rather than select it from the phone book.  In fact, when the
phone is in this mode, you can't access the phone book at all.  This is
not what I want.

Hopefully, I've managed to explain exactly  what I'm looking for.  Can
anyone please suggest a suitable phone?

Thanks, Peter