Determining the cause of a strange "the other side can't hear me" issue Recently I started to notice that a lot of people who call me, can't hear my voice during the call.

The phone is an almost 2 years old Sony Ericsson Xperia X10 mini pro, so I guess the microphone could just get old and somewhat start to fail.

Also it could be wrong from the beginning. I didn't had that much calls until recently. I think noticing it just now might be reasonable.

According to my experience the problem only occurs during incoming calls, I've never experienced it while I called somebody. And only in the first incoming call, if I was recalled right after the first call, everything was fine. This makes it look like if it is the provider's issue.

And of course some app blocking my microphone also came into my mind, but I don't have many installed and I don't know if any of them would do such a thing anyway. (The phone was never rooted, it has the official 2.1 ROM, etc.)

The question is: because I still have some of the warranty left... how could I determine if my problem is a hardware problem, a software problem or a provider problem to know where should I go with it? (According to my experience if I just say something is wrong without hard evidence, the repair service and the provider will only try to make the other take the blame. Not to mention that they will factory reset the phone, say that they couldn't reproduce the problem, so it had to be one of my apps.)
This doesn't really answer the question, so just for the record if anybody else has the same issue:

I don't know about other phones, but small Sony Ericssons have an interesting feature. They have small buttons in the corners of the screen. One of them is muting the microphone. And... as the user rises the phone to his ear, the screen that was turned off for the call turns on because the phone is moving, and the ear hits the mute button with an 50% chance. The other side swears and hangs the phone, so when I finally see the screen turning on again, the button and the mute sign isn't there yet.