Hello Dirco1907,
If we're understanding you right, you would like to use an external microphone with your iPhone via the Lightning port, while also still being able to listen to audio coming from your iPhone. We're happy to give you some information on this.
It's expected behavior for your iPhone to recognize that device as a headset. However, you can try connecting a Bluetooth speaker to your iPhone to see if you can get audio to play while still using your external microphone. You'll need to make sure the speaker does not have a built-in microphone.
Another option would be to use an iOS audio interface that would allow you to connect both your microphone and an external headset into your iPhone at the same time. While Apple doesn't sell this on their website, you should be able to find various devices for purchase by doing an internet search.
We also recommend contacting the manufacturer of the microphone to see if they have any apps or accessories you can use in conjunction with that mic to get audio to play like you would like.
Thanks for using Apple Support Communities.
Take care.
