Well it wasn't me :-) So how do you know, did you hear it ringing? Are you sure you didn't see it and accidentally press 'reject'? I think if you do that then there is no record of it.
Firstly ... have you tried fully powering the phone down and then switching it back on to force a re-boot? Then, does it do this with all call or just the odd one or two? Have you added or has the phone downloaded a new app? There are a number of things to check but a few details can narrow the options.