There's two views on it. The standard advice is have a cheap punting phone. Other punters like the security of a smartphone that can give them maps and access to many other things when a punt doesn't pan out.
I've tried both ways - when I started punting I got a payg sim and stuck it in an old Android phone that I had lying around at home from when I'd moved to an iphone, and used that - and found it very convenient to have it linking to a (punting only) google account, syncing my contacts automatically, and with some web access etc. on it; I could use it for my punting emails too so all communication was in one place, and with me. Sadly the speaker died on it, so I couldn't hear anything when I phoned. I replaced it with a cheap Nokia and used that for a while, but really missed the extras that the android phone had offered, and struggled to send texts on it, being much more used to the smartphone keyboard.
Then the chance came to upgrade my iphone, so I had the old iphone spare, so changed sims again, and now use that one, and have it syncing with my punting google account once more. It looks identical to my real everyday phone, down to ensuring it has the same wallpaper on it, so nobody even notices that I'm carrying it around. Just this week I was glad of it, as I was able to use the mapping on it to find the location of a punt, safe in the knowledge that it wasn't recording any of the searches on my real phone. I can keep any notes I want on it too.
When I don't need the punting phone, I can switch it off and hide it, and still update contact info etc. through the google account, knowing that it will automatically appear on my phone when I next switch it on.
But, everyone is different. This works well for me, but I can also understand why others would hesitate to use a smartphone for this.