People who are hard of hearing and use voice phones
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Voice quality can be very good
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VoIP to VoIP can have broader spectrum
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You can hear the S's and F's better for example
 
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Good quality components such as USB headsets also help
 
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IM is available during a call on some services
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If of course both parties are on computers or perhaps certain PDAs
 
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Negatives: Voice quality can be realllly bad, too.. No HAC computer USB headsets (?) so far