Mute button problems

[shepherdscove]

Hello, I have an iphone5 and have noticed the mute button keeps switching on, meaning I miss calls because there's no ringing sound! This has happened several times in last couple of days. I have the phone in a case so the buttons are covered (I can't see this making much difference though because if they weren't covered, the button could still
get knocked). I've tried looking online for solutions and there don't seem to be any! On the ipad I can change this switch to lock orientation, but that doesn't seem to be available for phone.

Has anyone else had this problem, and what can I do other than obsessively checking my phone to make sure I don't miss a call? I use the vibrate function but that doesn't help if the phone's in my bag!

Thanks