Not receiving text alerts when headphone jack is plugged in to iphone

I have posted this question before but a will post it again and rephrase it. When I am at a coffee shop or with a friend a put my iphone on the table. When a text message comes through the screen will not light up to say I have received a text message and it won't make an alert sound. This only happens when my headphone jack is plugged into my iphone 5.