Iphone 4 can't hear calls?

[Lewb]

Hello, could someone help i cant hear calls when i make or receive calls unless i use speaker phone started happening yesterday an haven't dropped it or anything like that an haven't ever even used headphone jack tried turning off an on an tried plugging in headphones several times also tried using compressed air but still don't seem to be working any ideas how to fix this? or should i take it into apple I've had it on contract for about 17 months so not too sure if its still in warranty have insurance with o2 tho should i take it into the O2 shop an see what they say? Thanks in advanced.