A conversation using a phone on your wrist is anything but private. . . and I hate people who use speaker phones in public. That is what happens even with the Apple Watch. It allows you to handle a quick phone call, but it is not intended for long conversations on your wrist watch. For those, you get out your iPhone and have a private conversation. For this reason alone, those stand alone wrist phones will be a fail. They will make their owners social pariahs. Here's an actual topical question for the thread. (imagine that!)
Let' say you have an iWatch. You answer the call. If you find it's a conversation you need to extend can you pull out your iPhone, and have the watch hand the call off to it? I would imagine it would, as the call actually seems to be processed on the phone and is forwarded to the watch via bluetooth.