Phone always switches non-preferred carrier (or roaming), but never back

Question

I've got a contract with 3 (Three) in Australia, which allows users to access the Telstra Next3G network if its own (pretty poor) network coverage is insufficient. The usage of Telstra is free for phone calls, but for data extra charges apply.
My problem is that even if I specifically choose "3" as carrier (i.e. not "automatic"), the phone eventually always switches onto "roaming" (which usually means Telstra). This happens I presume because in these instances my 3 signal is too weak. That's fine but it never changes back to "3" even in areas of excellent signal, so that I always have to either
pay extra charges for data downloads, OR
I change manually back to "3".
The latter is very inconvenient, since manual carrier changing takes 3-5 minutes of fiddling around (changing the carrier setting to s.th. other than "3", which brings a "no network" warning with the prompt to choose "settings", then waiting for the list of options to reappear, then changing to "3" and waiting for the change to take effect; all the while keeping the screen alive because if it goes to sleep no change will occur).
How can I either
lock my phone onto "3" when I choose this carrier option in the settings menu, OR
(if "automatic" carrier is chosen) make the phone automatically switch to 3 again when the signal is sufficient?

Answer 1

Hi NicYoung,
same behaviour here in Italy using H3G.
I normally go to settings and switch the phone to plane mode and back to normal.
That does the trick for me.
However, I wish it did this automatically...