Definitely get a simple punting phone, just like a £5 nokia from EE or something, that's what I use, saves so many worries when putting (don't do it very often now, can't find anyone I like in Aberdeen and money is tight with redundancy looming).
If you got another half, my recommendation is don't hide the punting phone, it raises too many questions that you can't answer, do what I do, I got it a few years ago as a 'festival phone', for taking to festivals and traveling abroad, she knows that's why I have it and it sits in the bottom of a drawer in the spare room along with loads of other cables and keys and the crap you can't throw away. I keep one of them 'sim converters' next to it, the ones you pop your tiny iPhone sim into, so that's for cover.
I keep the actual punting sim hidden away elsewhere in the house, not saying where!!, and just pop it in when it's time to use the phone, then take it out and hide it once I'm done.
I always make sure that I delete messages and call logs, and reset the phone when I'm done, just to be extra safe, that way if she finds the sim, I can just play dumb and say 'no idea, must be an old one or something'. Never happened yet but be prepared like the boy scouts say!!