Internal Mic on When connecting Headphones or Cable

Hi,

I'm having a strange problem with my iPhone 7.

I'm using it for playing with my guitar, and as I connect the lightning to 3.5 mm cable the internal iphone mic is still active causing lots of feedback and noise when using distortion.

How can i solve this?! normally when you connect anything to the lightning ,internal mic should deactivate..

Thanks in advance for your help

Salvatore.