I recently purchased a plantronics h171 headset for my snom 320 phone. I can hear fine but no one can hear me

Hear are the pairing instructions (see link below). I myself have a phone like that and the pairing worked fine.

Plantronics Explorer 230 Pairing Instructions

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ok..try to go your sound settings, maybe the microphone is turned off. OR try to restart your phone just to make that headset is working properly.

I would suggest deleting the headset from your phone. Turn your phone and headset off. Turn your phone back on and follow the directions to pair the headset to your phone. I will include the link to the user's guide below.

Plantronics Explorer User's Guide

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We've never dealt with snom wireless adapter but the CS70 and HL10 lifter is a popular set for your phone. there are a couple of things you can try to cut down on the echo.
1. Make sure CS70 is proper distance from phone, minimum of 6" (12" from a PC)
2. Make sure round compatibility dial (located on left side of base) is set correctly. Most phones work on setting #1 (never between numbers)
3. Echoing is usually caused by an improper mix of incoming and outgoing volume. With a friend listening in you can make broad and minor adjustments for both (incoming: back of base (1-4) and +/- on headset; outgoing: bottom of base (a-d) and +/- arrows on back of base).
Following these steps should help you out.

Good luck from Headsets Direct - 800-914-7996.

You need to remember a couple of things when using amplifiers and headsets. Normally they will not work if you connect them to a phone with cordless accessories. Also, you need to make sure that you're connecting the amplifier to the HANDSET jack on the phone (this requires removing the handset off of the phone to access an open line). Give that a try and you should be fine. If not, contact your retailer or Plantronics at 800-544-4660 x5538 for warranty service.

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Plantronics always recommends that the old phone is taken out of range of the Bluetooth headset when trying to pair it to a new phone.

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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.

This may be a broken or loose wire in your headset that prevents you from hearing them.

Or the headset is not turned on. Press the call control button for 2 seconds until you hear a beep or see the indicator light glow blue to turn headset on. Your headset is out of range. Move the headset closer to the phone or Bluetooth device.

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http://plantronics.com/north_america/en_US/products/mobile/bluetooth-headsets/voyager-510