For business customers, they could be talked into numbershare between a flip phone and a smart phone - but I think I was paying for two phones.
The best thing for my productivity is my current set up.
I have an iPad mini, and a flip phone, and my numbers on "number barn". We will be adding some of google voice's number for business to some work stuff. I might put one on the iPad. Not sure how well the PBX feature on GV works, but it works just fine at being able to make calls, etc.
Frankly, a phone is a piss poor compromise, IMO. Something much smaller, and useful for texts/to see vital alerts/get taxis/act as a wallet, makes sense.
After getting used to my mini, I cannot imagine going back to dealing with a phone.
Now, as to why Apple only makes their pro line, in the larger tablets/why they haven't thought about the most common notebook size being an A5, I don't know,