I installed duo on my wife's iPad. It asked for a phone number so I used my wife's android phone's number. Google sent a text with an activation code to her phone. I inserted the code into her iPad and duo worked fine on her ipad.
She can call out with duo from her contact list. I'm not sure what will happen if I called her iPad with the phone number from her phone. I'm going to try it one of these days. Given it was activated with her number, I wouldn't be too surprised if it works. After all it's not using the voice network at all - just wifi or data so the phone number is nothing more than an identifier.