Tel no format
I constantly had a problem with my stored numbers changing format. Sometimes I was having to correct this 2-3 times daily. I have an iPhone 11 but also had an iPad mini that only had iOS 12.5. However I recently bought an iPad 10.2 and since the iOS on my iPad and phone are now the same my telephone number format has remained static.
Not sure if this is the problem solved,but certainly works for me now.