Ok, it looks like I solved the problem. After hours struggling with this today, I found out what could fix it.
I went to:
Render settings>GI settings>General>Samples and increased the value to 200%
Render settins>Options and turned identical noise distribution on
Render settins>Physical>Blurriness subdivision = 4
Shadow subdivision = 4
AO subdivision = 4
Surface scattering subdivision = 8
It has improved a lot the noise on the back of the phone, but hasnt removed it completely.
Thanks all for the help.