There are sim card copiers out there so you can "Back-Up" your sim card and its phone numbers to another card. Also the second-third-fourth card will work in other phones just not at the same time. Each time you want to use another phone you have to make a call from that phone to lock in that sim. Otherwise if you just turn off one phone and turn on another with a different sim, any incoming calls will be routed to voicemail until you make a call from that phone. I have not done this myself but have done some research on the subject for probably the same reason you want to have two phones. I want my pda phone for work and a slim phone for when I want to just have a phone with me. Swapping sims is a pain in the butt so I researched the multiple sim options and that is what I came up with. Also, there are sim cards that have the ability to have multiple accounts on them but the phone needs to support such a feature. It is basically like having a two line phone. I know that nextel used to have that feature built into them but I don't know about that anymore. You could have two different phone numbers and phone lines in one phone but they both had to be from the same area code.