My new Siemens S65 gave a lot of trouble.

I was triyng to connect it to the PC via Bluetooth, but the PC software (Mobile Phone Manager) wouldn't detect the phone even though it was successfully paired! Then I tried connecting with the DCA 510 cable -- the same happened! I was beginning to think my phone was malfunctioning. Also people to whom I talked on the phone said that the sound was very bad and the volume was very low...

Then I happened to switch off GPRS (I had the option on) and all the troubles went away :)

Now I can connect it via bluetooth (hadn't tried the cable yet but I bet its working) and the sound is back to normal.

Moral: Don't switch the GPRS option on! (I still don't know if that option keeps the phone from using GPRS when needed or if it is just to make it permanently connected via GPRS).