I have Cingular's 8700c and the Motorola BT H700 worked briefly but now I also experience the drops after exactly 5 secs. Or rather the call drops back to the BB and you can hear the tiny voice coming from the handset; meanwhile BT headset gone quiet although apparently still connected.
Tried all options on the Device options on the Blackberry side (trusted vs not, echo control y/n, encryption y/n), pairing with a large vs small distance between headset and phone.
Nothing seems to help. So I have currently given up and tether my ear to the BB via the trusty wired headset.. looking so twentieth century
)