Change the default Radio Band

I need to change the default radio band.

I change the band by dialing ##4636##, I change it to one of the bands in the list that is recommended, and the phone works fine.

However, for some reason sometimes it gets reset to its default (factory setting?) band,
so every times that happens I have to do the setting over again.

Is there any way to change the phone's default radio band setting?
(maybe by editing an OS setting file, etc.)

Thanks

Thanks, but it seems MTK engineering mode doesn't work on Xperia as expected...