I know for certain that none of my other phones did that.
It should have worked on other phones. Plugging your phone into the auxiliary jack is just like plugging in a set of headphones. You're just using your car stereo (as an amp) and the speakers instead of the headphone speakers.
Difference with the Eris is you were using it at the time as either an MP3 player or playing Pandora or I(Heart)Radio... which allowed you to discover this feature. I'm guessing your old phone either didn't have an MP3 player or if it did, it blocked calls while you were using that feature. But trust me, if you had plugged in a flip phone or basic phone in and used the Auxiliary button on the radio for a call it would work (keeping in mind that you'd still have to have the phone open so the mic. could pick up your voice.
Now another great thing to keep in mind is if you get a new car (or radio) with Bluetooth capability is that you can match that to your Eris. Then all this can be done wirelessly instead of through the auxiliary jack.
Even without 2.1, I still love this phone.