My phone is not transmitting I have an Htc Desire and I'm using the headset that came with it. During a call yesterday the other party stopped hearing me.  I unplugged the headset and continued with the conversation normally. Today I checked the headphones again before buying a new pair and indeed the other person couldn't hear me. What was really surprising is that I could hear myself,  echo with about a second of delay. So, electronically my headphones work. Is there something broken in my phone? 

Update: I just tried the Skype test call and it worked just fine, so it seems to be my headset when making phone calls, which is really weird.
Well, that's hard to say. Its either your phone, your headset or both of them. I'd say: go to a shop, ask to test a new one. If they work: buy them, its just your headset. 

If they don't work: Test your headset in another phone. If they work: it's your phone. If they don't: its your phone AND you headset.