IPhone 5S manual network selection

Question

Hi all,
I have an unlocked iphone 5S and seem to have an issue with the manual selection mode. I was travelling outside my resident country and switched to manual network selection mode to choose my preferred roaming network in the foreign country. After spending 10 minutes scanning, the phone didn't come back with any results although there was clearly network coverage of my preferred roaming network where I was standing. I had to restart the phone and then surprisingly ios 7.1 changed the network selection mode from manual to automatically. After changing it back to manual, I was finally able to get my preferred network. However, there was another issue with this - after successfully registering onto the preferred roaming network and having made calls on this network, the phone for some odd reason switched to another network which I hadn't chosen. This happened despite the network selection kept on manual. Eventually, my roaming bills went through the roof due to this.
Can someone pls help out with this issue?

Answer 1

Just to clarify the original query.
1. I manually select, say, three Sweden when I'm in Sweden.
2. Just so we're clear, I have tried switching off and then re-booting the phone in manual mode.
3. I do whatever mail sending etc is necessary only to find the phone has flipped over to, for example, 3G Sweden, which is not part of the Three Group.
4. It seems to me that in the midst of preparing a message etc, the network goes down and the phone then picks up the next strongest network signal in the vicinity, even though the network selection is set to one network only. And therein lies the problem - the flicking over and back costs money.
Three.ie blame Apple, but why do I have a sense that it may not be all Apple's fault, that there may be a network switching issue mixed up in it as well?