K300i no sound from receiver

Hello,

I've got a bit of a strange problem with my K300i. It was working fine
up until a week ago. I received a voicemail on Sunday and went to
check it only to find complete silence when I connected to voicemail.
Thinking my voicemail was broken, I was going to contact Vodafone, but
someone tried to ring me today and I couldn't hear them either. When I
thought about it I realised I hadn't heard the usual dial tone when
calling voicemail. In other words my receiver is completely silent.

Any ideas on how to fix this and what could cause it? Very odd and it
makes the phone completely useless, other than using text messaging.

Regards,
Gordy.