A780 - Network selection

Hi,

I've been looking for a way to change the network of my A780, so when
I'm roaming I can select the network I want to roam on (cheapest one).
Motorola is not known for it's brilliant phone menus, which comforts me
in a way because I can't find the auto/manual network selection in the
menu... The manual is also not helping, there's no part about changing
networks. For now, the phone just seems to pick whatever network it can
find ... randomly.

Who can tell me where to find it or force it to change networks?

Thanks!