Unlocked N1 with T-Mobile not working overseas with foreign SIM. Help!

[ahmerjaved]

So I bought my Nexus One a couple of months ago. I especially bought the UNLOCKED (compatible with T-Mobile) for $529 because I had anticipated a few months abroad and wanted to use it freely with my foreign SIM. Been using it for 2 months with my T-Mobile family plan, and no issues!

However, just reached Karachi, Pakistan... and put in my Pakistani SIM card but it simply says "no service" when I pull down my notification bar. When I go into settings and try to search for mobile networks, it gives me an "error while searching for network."

Why do you guys think this is? I thought the whole point of the unlocked phone was that I could use it everywhere; just put in my SIM and get on with it? Could this be because of difference in cellular frequencies? I'm not very techy so bear with me! Is it possible I need to ask my service providers to issue me a specific SIM or something?

If you guys need to know, I have two SIMs with different providers, and I have tried both with the same result. Here's some information on their frequencies:

(1) Mobilink........... GSM 900/1800 MHz
(2) Telenor........... GSM 900/1800 MHz and UMTS on 2000 MHz.

That is Greek to me, but you guys may be able to make sense out of it. Please help me out and tell me what you think the problem is, and how I should tackle it!! I have to spend another 3-5 weeks here and I want to use my N1! Thanks.