No. It's not a cell phone. When you are tethered to your LG (I have one, too, a Shine), you essentially using the data portion of the phone as a modem. If it's a 3G phone, a fast one...

To use your 810 to make cell phone calls would require some fancy BT streaming, both at the phone and at the 810.

As others have stated, you can use VoIP through the 'modem' connection. I have an Asterisk PBX at my house, and my n800 connects to it whenever it's got a 'net connection, either WiFi or 3G, @home or out and about...