Iphone 3gs can't hear caller, but speaker and headphone mic work when pressed

Hi,

I know it's an old phone, but today while I tried making a call, I noticed there was no tone while calling, however the person I called picked up, I pressed speaker and it worked. Also when I used my headphone+mic, it also works but it's a hassle because I don't want to carry my headphones wherever I go.

Any way to fix this without spending money? I'm waiting for the new Iphone to come out, so I'm hoping to continue using this phone till christmas or so.
Please help, it's really annoying!

Thanks,
Luis