No service in roaming with iPhone 5s / iOS 8.2 (12D508)

Sigh. Let's take this slowly. Yes, there's something wrong with the phone, if there wasn't I wouldn't ask for help here. Duh. If it helps to understand the problem, the same Colombian SIM works fine in an S4 in Chile, i.e. it stays registered.

However, the "something wrong" is a software problem, not a hardware problem (the iPhone 5s works fine with the same SIM in its home network in Colombia, and it also works fine with a Chilean USIM in Chile).

There's nothing wrong with my mobile account. If there was, do you think I would be able to manually register with the Colombian SIM in Chile? (I didn't mention it, but while the iPhone is registered, I can make and receive calls if I'm really quick, before it decides it's time to deregister again).

I know a little bit about mobile phones and networks, and I'm hoping for a higher grade response than this one.

Apologies for posting this again, I hope the troll doesn't bite this time. I realise that this is a fairly specific question, but I would be very grateful for any real help with this problem.

I'm using an iPhone 5s with a Claro Colombia SIM card (not a USIM) in Chile. I switched off data roaming, I'm only interested in voice roaming at this point. Just to emphasise in advance, the iPhone is perfectly functional in Colombia with the very same SIM card, and it also works in Chile with a Chilean USIM. My account is fine, roaming is enabled with Claro Colombia and I do not need to contact my carrier about this. It's an iPhone problem. Really.

Roaming is enabled with the Colombian operator, and I can register manually with Claro Chile.

However, after a variable time, several seconds to several hours, but generally after a few minutes, the iPhone loses service on Claro Chile and remains in "No Service".

It never recovers service unless I register manually again. Automatic registration does not work at all.

I reset the network settings, then all settings, but there was no change in behaviour.

Can anyone help? Showing "No Service" with a valid SIM and a valid network is a showstopper bug, the iPhone shouldn't be type approved with this behaviour. If I could, I'd open a RADAR ticket but that doesn't seem to be possible.

iPhone 5s, iOS 8.2 (12D508, Carrier Claro 18.0, modem firmware 3.11.00)