Testing an unlocked phone
 

Will b in UK for 3 months. Bought an unlocked GSM quad phone on ebay. I plan to buy the SIM card when I arrive in UK. But to test the phone, can I just "borrow" a sim card from another GSM phone?
From what I am reading here, seems like the best plan is to buy SIM in UK.
Also trying to sort out the best network. Cell will be mostly in case of emergency back in US and calling ahead within UK when landline not available. Any suggestions?
Virgin Mobile?

That's how it works. If your phone is unlocked, it will work with the other SIM (and if it isn't you'll get a message like SIM LOCKED or SUBSIDY CODE, in which case you will have to unlock it, which isn't usually hard to do).

Of course, if you want to use mobile data or send picture messages (if it's a camera phone) you might also want to add the settings of the network provider you choose.

This would be correct. You can get them anywhere in Britain, and they are sometimes very cheap.

What, now in the 21st century? Once you have a mobile phone, why would you want to use a landline to call ahead?

Virgin Mobile is a good all-rounder, and I would definitely recommend it if you plan to return to the UK -- it's easy to keep it alive, a text message every six months or so is all you need (and it can roam in the US and elsewhere, so it's always useful as a UK number or emergency phone).

Otherwise, Carphone Warehouse has an international-calling SIM that has 5p calls to the US. However, it doesn't roam, and it will expire without activity after 6 months.