Tri-Band is simple. The phone is able to use the 900 GSM system (Vodafone and 02), the 1800 PCN system (Orange and T-Mobile) as with most modern Dual Band phones, but it can also log onto the American 1900 system on demand. The phone will seamlessly chose which frequency it needs.

Remember that a UK phone will always be a UK phone. So while you are abroad, every call you make will effectively bounce back to the UK and be charged as International, even if the person is standing right next to you. Also, you will be
charged for Incoming calls.

I would think that if you are going to be there for 18 months, then you would be better to buy one out there.

Hope this helps.