Yes, in theory, you could use your "cell phone" (actually just a WiFi tablet, for this use case), but it wouldn't be using either of the two types of OBi dongles. There are several methods, including Obion (sucks) or a softphone app like Counterpath X-Lite or Zoiper. However, in real-world use, you aren't going to be satisfied at all. You'd need a strong WiFi signal, and you would get rather poor battery life, keeping the WiFi/VoIP connection "awake" all the time. And, then there's getting your greasy hands on the phone and trying not to drop it.
The better, and frankly obvious solution is to simply plug a modern DECT 6.0 cordless telephone base station into the OBi, and take the handset outside. DECT has much greater range than WiFi, is designed to be used while moving around, and has far better battery life. You can either use what you might already have, or buy a new one at your favorite big box store for not much money.