Try to hard reset your phone it may be a software fault.
But it can also be a hardware issue so if the hard reset doesn't solve the problem then take your phone to a hardware expert.
SOURCE: How do I change the
To set up voice mail, perform the following steps:
1.
To open the phone pad, press the green Send 
key.
2.
Tap and hold the 1 key.
NOTE: A tutorial will begin.
3.
Follow the tutorial to set up voice mail. The tutorial will guide you in setting up a password and recording your greeting and name.
NOTE: The default password is the last four digits of your mobile number. Your password is required when accessing voice mail from a landline phone or when you are roaming.
Source:http://support.t-mobile.com/doc/tm22912.xml?docid=1738&referring%20topicid=73&A2L.SERVICE=&Referring%20TopicID/DocID%20List%20Index=y&navtypeid=2&pagetypeid=20&prevPageIndex=1
SOURCE: Why can I not hear the caller talking when I answer my HTC 7windows phone
This is fault on your earpiece and needs to be replaced ip you can remove the panel and confirm if the earpiece has a continuity or if there was any connection issue.
Finally test using a handsfree to confirm that the board is working well and the fault relates only to the earpiece.
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