Questions about Apple's Bluetooth headset

Someone recently gave me their old iPhone bluetooth headset (the kind that Apple made but discontinued) and I have some questions about it. I have my phone set up right now in the headset's dual dock with a cable connecting it to my speakers, so that I can hear it ringing throughout the house. Since it's usually just sitting there when I'm home, I'd like the headset to stay connected to my phone so that when a call comes through, I can just pick the headset up from the dock and use it to answer the call. I had no trouble connecting the phone (a 3G, by the way) with the headset, but whenever the two are connected back to the dock, the connection between the two is ended. I'm not sure why and really would like to prevent that. Any ideas? I don't have the original manual to help me and while I did look at the manual online, it didn't really help me with this.

Edit: oh and I know the headset dock isn't originally compatible with the 3G but I did some modding and filed the sides down so it fits in nicely