Hi Jodog:
When you say "the caller's voice is very weak", do you mean it is weak (low volume) when you are hearing it through a Bluetooth headset? If so, there are several things you can do to try and solve the problem:
1) See if adjusting the volume control 'UP' on the side of the iPhone makes any difference. It is possible that the phone remembers different volume settings for different speaker arrangements (built-in phone speaker vs. external speaker paired by Bluetooth).
2) Check the 'MIXER' setting on the Zumo 595. You can't control headset volume with the volume controls on the Zumo 595, but there is a 'MIXER' control on the 595 that allows you to adjust music volume & navigation system volume independently of each other. My guess is that phone volume would be controlled by the navigation system volume. Play around with that a little bit and see if you get an improvement.
3) The Zumo 595 supports two different types of Bluetooth connection, one is A2DP, the other is an older standard less suitable for music but in general more compatible with a broader range of devices. You might want to check the documentation for your Bluetooth headset and see if it is designed for A2DP. If not, that might be the cause of your problem.
There is also the tried-and-true troubleshooting technique of un-pairing the phone from the Garmin (telling the phone to forget the Garmin), unpairing the Garmin from the phone (you need to tell both devices to forget each other), and finally telling the Garmin to forget it is paired with the headset. Then pair the three devices up again, paying close attention to the sequence of pairing given in the Garmin manuals. There are different ways of pairing the devices, depending on which ones are multi-channel and which are single-channel, and the sequence in which you pair them does make a difference to functionality.
Lastly - don't forget to consider that you might be experiencing a degradation in the performance of your Bluetooth headset system - in other words, that system is failing, and the iPhone and GPS are both working OK. You can test for this by pairing up your phone and GPS with someone else's helmet Bluetooth system and seeing if it works OK, or even by pairing up your phone and GPS with a conventional (meaning, non-motorcycle-helmet) Bluetooth headset system.
Michael
