Charged for call I did not make

I was looking at my bill and I saw a number of calls I was being charged for that I did not make.<br> I know this as I never use the landline to call cellphones as well as I do not who the number belongs to<br> I only use my landline for other landlines as well as i only use it when it is free, eg weekends or at night<br> Please credit me before I do a payment for the account