It is going to be one of three things...Hardware, Software or your network.
It sounds as if you have already attempted the hardware fix, just make sure you have installed the Mic portion of your port assembly correctly and may even try to use a very small piece of cotton (Like from a Q-Tip) and pack it around the mic.
It could be a software issue, you could back up your data on Itunes, restore it and see if that helps
or...call your network provider and explain the issue. they may (or may not) be able to help depending on who you get on the line and how willing they are to try and help.
I am not talking from experience, as I have not had this issue...but did research it a bit to try and offer some help. If you do figure it out, please post back what worked for you to offer a fix for others that may have your same issue.