It shouldn't have to be re-paired...
I have a Motorola Razr V3c (Verizon)which connects to the car every time I get in. I had planned to get a car kit but the phone works so well in my pocket I don't think I'll get the kit unless I find I need to recharge the phone. Yours should be the same and should pair up and remain paired.
That was going to be my suggestion too...
I would check the settings on the actual phone. On the E815, go to your Bluetooth menu, then to Setup > Power > On. It sounds like you may have the power off, only even if that were the case, the phone shouldn't have to be re-paired each time (it would only need to go through the "Find Me" process).
If none of that works, you could have outdated or defective software on your E815. I know that my wife had her V710 replaced once because they couldn't get the BT functions to work. She now uses the E815 and hasn't had any problems.
Good luck. Bluetooth is really a great feature. I've had the E815, a Treo 650, and a RAZR all connected to my car and they've all worked just fine. It can be done!<small>
'05 BMW 745i Sport (TiSilver/Black)
'06 BMW 650i Coupe (TiSilver/Black)
'06 BMW 325xiT (SpGraphite/Beige)
Blue Tooth works perfect with my Motorola in my M6
Took a little learnin' to get me there but once I learned it was no problem.
'06 ///M6 Silverstone
'02 Acura TL-S
'02 ///M3 SMG
'01 ///M3 6sp
Dodge Stealth TT AWD
Acura Legend Cpe.
Piper Cherokee 180
Datsun 510 2dr
MGA Mk.II (<400 manuf)
+ Ford, Mercury, Lincoln, Chev, Buick, Caddy, Plym, Dodge, Chrysler, Toyota, Nissan, Honda, Acura, Mazda and other boring stuff for wife and family.
Yes, he may have chosen "temporarily".............
...the first time they went to pair the phone, in which case it will lose the connection.