I'm assuming that you've already done all this, but possibly not:
1 Put your headset into pairing mode.
a With the headsets boom closed,
press and hold the Multifunction
Button E until the headset
indicator turns on (about
3 seconds). The headset indicator turns on.
b Release the button, then open the
headset boom. The headset indicator remains on.
2 Perform a device discovery from the phone. For details on device discovery for your phone, refer to the phones users guide. When the scan is complete, the phone displays the devices found.
3 Select the Motorola HS850 from the discovered devices list and confirm following the on screen prompts.
4 When prompted by the phone, enter the passkey 0000 and confirm to pair the headset with the phone. The device ID is now stored in the phones memory. Pairing is successful when the headset indicator flashes 10 times rapidly.
And for the part that
might actually be the issue:
Your headset will only automatically initiate a paired connection (by opening the boom) with the last device it was connected to. If you want to connect your headset to another device in the paired device list, you must initiate the connection from that device. Whichever of the eight devices initiates a connection to the headset afterwards, that device then becomes the last connected device.