Samsung Sgh-E900 Mobile

[val-south]

When I am trying to dial a number "unlock" appears and no sound. Sometimes I can hear the 'phone ringing and I can make a call, but other times nothing. I have not consciously locked the 'phone (in fact do not know how). The last person I tried to ring said she could hear her 'phone ringing but could not speak to me.

I am a very low level mobile user so have only limited knowledge of mobiles
Any help would be greatly appreciated.

Thanks