What I hope this is is the ability to go running and have your smart watch act as your phone. Get in your car and have the car stereo act as a phone. Then switch it back to the mobile phone device. That's something GV does not do or if it does it requires wifi.
The way I understand this to work is that each device's IMEI can be added to the pool for a particular number and you can select on the fly which device(s) are active. In addition they are offering that you can have multiple numbers and multiple IMEI's pooled between them.
The only thing surprising about this is why, in 2017, this hasn't happened sooner.