On the phone gps accuracy subject and speaking as a non-technically minded person, I'd trust the gps on a dedicated unit before a phone.
I remember testing out various gps apps on my phone utilising different apps and recall one situation very clearly.
On the Dutch side of the Schelde estuary my Motorola phone wrongly showed my position on Osmand and Google Maps, first with data off, then on. I was still in Belgium according to the phone, yet not in a location I had been.
My older cheapy Chinese phone showed my accurate location on Osmand and with neither saved Google Maps nor a sim was useless for Google maps.
In any case, the longer the gps is activated on a phone, the more accurate it is, I have found.
I've also noticed that weather, specifically thunderstorms, have an effect on accuracy on all the gps devices I've tried, although not to the extreme that it has me in another country
If I ever have the suspicion that the location may be inaccurate, a short ride or walk normally causes the phone to recalculate.