My iPhone thinks I am in an abroad network (but I am at home)... What to do?

Hi,

I am having the rather funny but annoying problem: After returning from abroad (Thailand) to home (Australia), my iPhone 4S "thinks" I am still in Thailand?!

This surfaces at two points:

(1) Calls to Australian mobiles do not go through, with network voice errors such as "Optus advices the number has changed", or "There is no such number in the country you are calling". If I add the country code for Australia to the mobile number (which I should not need to do, calling from Australian mobile to Australian mobile in Australia) some calls go through, others still won't work.

(2) Locations on Google Maps app (I am on iOS5 and would prefer not to upgrade to iOS6 for performance concerns) are shown as "Sydney Opera, Sydney 2000, Thailand". The GPS location itself is shown correctly.

What is also confusing, is that (1) seems to be network side, while (2) seems to be phone side (right?)

Anyway, any approach to fix this (preferably without upgrading iOS) would be much appreciated! (I have already set all international settings back to Australia, restarted and resetted the phone. This has not helped.)

Cheers

p.s. Provider: Optus via TPG (in Thailand I had a prepaid DTAC SIM card). I emailed my provider, but they have not responded, and also I am not sure that the problem is actually on the network side.