I have a Plantronics PN 66935-05 and am trying to connect it to my work phone. I have it plugged in but can not hear the person on the other end of a call

SOURCE: Volume control

the ear piece may be dead or if its a flip phone check the flex cable

SOURCE: cannot hear ringing tone or voices

Its the problem with the speaker... Change it...

SOURCE: I can not hear anything on my pearl when I answer the phone

when in the call push the volume button on the side of the blackberry

SOURCE: plantronics CS55. When using the wireless headset

Echo is usually caused by the transmit (speak) level on the unit being set too high.

1. First, try lowering the volume of the telephone. If you still hear an echo, adjust the volume of the headset itself.

2. The master transmit controls of the headset are the ABCD switch settings on the bottom of the base. A is the lowest and D is the highest setting.

3. You can fine tune this setting with the plus (+) and minus (-) buttons on the back of the charging base.

4. If this doesn't fix the echo, try moving the configuration dial on the left side of the charging base. The recommended setting for most phones is 1.

SOURCE: I can hear but the person I am talking to cannot hear.

try cleaning the mic' without openning the phone -
from the outside by an old touth brush...
that will do the trick

if you can open the screws without any damage
than pull out the mic's cover
it is probebly full of moist