How do I convert the S12 Plantronics headset to the over-the-ear headset. The instructions are very vague?

The S10 amplifier/headset system sets up like the Plantronics S12 system of which the user guide is included in the link below. As you will notice, it is designed to work with a telephone. It will not work as a stand-alone unit. To connect it to a dsl box, magic jack or any other line you will need a phone.

Plantronics S12 User's Guide

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S12 system - Some phones have jacks for headsets but the S12 is designed to connect through the HANDSET jack of the phone. Make sure all the cables are connected properly on the amplifier and telephone. and that it's configured (compatibility slide switch located on side of amplifier) to your phone. Once connected properly remember that you will need to lift the handset in order open a line on your telephone.

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I am giving a couple of links that may help you.

CS50 Reset

CS50 Information Booklet

CS50 Subscription/Pairing Instructions

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Unfortunately, the phone does matter. The S12 is designed to work with office style phones where the handset is just the mouthpiece, all other features located on the base. My suggestion is to get a phone of that style (you can find inexpensive ones at Wally world) and connect the headset following the instructions on this link for the Plantronics S12.

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Plantronics has a very nice of ear tips for the CS70 that includes several sizes, so you can find one to fit your ear. Here is a link where you can purchase the set of ear tips:

Plantronics Replacement Ear Tips

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Well it looks like the best solution would to buy another headset assembly. You could short out the whole system if you do not. see link below. It's @ 70 bucks but look on eBay and you might find a good deal.
http://www.plantronics.com/north_america/en_US/products/home/home-headset-accessories

The link below of the user guide for the Plantronics Voyager Pro has the instructions for removing and attaching ear buds.

Plantronics Voyager Pro+ User's Guide

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The round portion that has the smaller hole in the center, with the small extension, goes into your ear. The squarish loop then needs to be rotated to out, so that it is "hooked" into the curve of your ear. Try different sizes to get the right fit of the loop (the bodies are identical).

That is the job of the ear piece to keep the device in the ear