I found that having a separate punting phone was worse - a former GF found it and asked questions about why I would have a much lower quality phone than my regular one, and to be honest I didn't really have a good answer. LUCKILY I'd deleted all call, text history from it so it just looked like an old phone that I'd forgotten to chuck away/recycle. But it was a tricky conversation, made worse by the fact that I was clearly shitting myself that the truth would come out so must have been acting really suspiciously.
Now I use several layers of cover: I upload an entirely separate browser before going onto AW/UKP, always use private browsing, get the numbers I need, then delete the entire browser, so there's basically no record. Then delete all call/text records and number stored. 5 years in (ever since ditching the second PAYG phone) I've never been caught, and I've never failed to do all the necessary deleting. Also, I never punt on a day that I know I'll be coming home to the current GF - I only punt when I'm either away for work for a night (which I am every couple of weeks), or when she's away (which is less frequent but happens maybe once every couple of months). That avoids the problems of perfume, long hairs of the wrong colour, unfamiliar-smelling shower gel etc.....