Used NZ Treo in USA, Now Won't Find Network in NZ

[chokcake]

Hoping someone can help as this seems to be a Treo problem - but its a little complex.
I live in NZ and bought an unlocked Treo 750 here and have been using it with a local (Vodafone) SIM. I was in the UK for 3 months recently and purchased a UK Vodafone SIM which worked fine in the UK and Europe. I had a 24 hour stop over in the US on the way back to NZ and the phone, with the UK SIM in it, was switched on and linked with the US carrier (AT&T - I think - didn't pay a lot of attention as I didn't make any calls).
Got back to NZ and replaced the UK SIM with the NZ one - and hey presto the phone is no longer able to find any service provider! This problem occurs with both the NZ and UK SIMs in the phone. Its not a SIM problem as they both work fine with another phone.
I have just spoken with a friend who also has a Palm Treo 750, bought in NZ and she experienced exactly the same problem after she returned with the phone from the US (she used her NZ SIM on global roaming over there) and apparently so did another member of her company.
I suspect that connecting with the AT&T network in the US has somehow locked my phone to that network. Does anyone know how to fix this problem?
(Have tried contacting both the NZ Vodafone network provider and AT&T but what a surprise - no-one thinks its their problem and am getting no where with them.....)
Thank for any help - I'm getting desperate!

[jfa]

Using an att sim card shouild not lock your phone to ATT just bc you used it in the US. Have you done a soft reset after putting the NZ sim card back in?

[tracytresidder]

I had a simliar problem. Do a soft reset and then go into settings, phone, services, band selection and choose one that has GSM (900+1800) +UMTS 2100. I think thats the one - if not high light one of theothers and see if that works - usually does for me.