What to check if Plantronics cordless mic does not work but earpiece works fine

There are a couple of things you need to check. First, make sure it's connected correctly to the phone. This guide should help you with that:

Plantronics CS70N User's Guide

Secondly, the compatibility dial on the left side of the base has numbers 1-4. The indicator to the right of the dial needs to point to one of the numbers, never between numbers. Most phones operate using #1 for compatibility but you can dial in the others if you're still having issues. That should get you up and running.

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Two things to remember. If you're connected through the handset jack, the handset (receiver) needs to be lifted off the phone to get a dial tone. Secondly, if you're connected through the headset jack, you may need to adjust the configuration dial (round dial on left-hand side of base) to make it work when the headset button on phone is pushed.

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1. Check the configuration dial on the left hand side of the base to see if another setting works better. The recommended setting for most phones is 1.

2. Make sure your speaking (transmit) level is not too low. First, try increasing the volume on the telephone. Secondly, the master transmit controls are the ABCD switch settings on the bottom of the base. A is the lowest and D is the highest setting. You can fine tune this setting with the Plus (+) and minus (-) buttons on the back of the base that are above and below the picture of the microphone.

3. Double check the battery has a full charge. To check press down on the headset into the base station to ensure it is in the base in the correct position.

The Amber Charge light on the base will flash while charging and will stay solid amber when fully charged.

3 hours = full charge

Check the telephone configuration dial: Refer to this picture to see what I'm talking about: http://headsetplus.com/images/cs55_controls.jpg
Make sure this dial is on 1.  Most of the time 1 works, however if it is already on 1 and you're still having problems, twist the wheel around through the different numbers and try this.
Have you tried turning the major sound adjustment up?  It's on the back of the base.  4 is the lowest setting and 1 is the highest.
Finally, try changing the volume on the phone itself.  One of these should work.
One last thing, make sure that the cord coming from the back of the base is plugged into the handset jack of the phone, not the headset jack, and that the coily cord coming from the phone's handset is plugged into the front of the cs55 base unit.

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.