Mute function is toggeling on and off

Hi! I have had my iPhone 3G 8GB for about 1,5 year now and I have had no issues until now. I am not sure if it was after my son dropped it or not but the phone is toggeling the mute functions if pressure is applied to the left side of the phone, close to the mute/volume buttons. Sometimes when my iPhone is on the table, small vibrations could trigger the toggeling. Along with that comes the vibrations which confirms that the phone has entered silent mode. This leads to a very irritating cycle and it's very battery consuming as well. Has anyone had this issue and is there a solution? I am most grateful for any help.