If somebody have a problem with iPhone carrier settings update on jailbroken iPhone:

“iTunes could not update the carrier settings on your iPhone. An unknown error occured (0xE800003D)”

Then, you might want to try ssh to your iPhone and change permissions in the /private/var/mobile/Library/Carrier Bundles/ folder.

Something like

chmod a+rw -R /private/var/mobile/Library/Carrier Bundles/

worked for me.