Why can't I talk for more than 90 seconds on my phone?

[Peter]

I have a sony ericsson w580i that I have had for about a year. It worked fine until just lately. Three of the last four conversations I have had, after about 90 seconds it will only let me listen and not talk. I had a phone conversation at work (appx 20 miles north of home in a larger town) of about 5 minutes in length that went fine. The other three calls were at home and it will only let me listen after about 90 seconds go by. I have never had reception problems before and tried it with and without the headset, but it made no difference. I think if I call AT&T to get an answer, it will be too long a conversation and the tech on the phone will be unable to hear me. I think I did everything like usual with the phone and have no reason to believe it is damaged other than the one mentioned above. Anyone have a similar problem or possible solution?