Sound cracking during calls

Hi iFixers !

I have a strange problem on my iPhone :

When I call someone, he can hear me speaking. But when he's speaking, instead of hearing his voice, the sound is cracking, as if there is a lot of interferences.

I'm quite sure it is a logic board problem, because when I press on the bottom of the logic board (sim card, or speaker connector, the voice reappears and I can hear him well.

Sometimes, the phone looses my sim card (no Sim), but after restarting it, it find my sim card well.

It looks like something is desoldered, but I wonder which part it is. Can somebody help me ?

Thanks