If you have multiple phones can you just take the SIM card and put it into the other phone and have it work? I realize that the ESN is attached to the phone itself, and each phone has a unique ESN, but if you activate a SIM card, is the SIM registered to the ESN on the phone or can you just pop it into another phone (this is all assuming we're on the same carrier).
I apologize if this question seems elementary or confusing, I'm still learning about how GSM works. I'm looking at buying the 8125 (currently on Verizon, and looking to switch to Cingular) and would also like to get the new 3125 when it comes out. If I wanted to use the 3125 one day as opposed to the 8125, could I just switch the SIM card from one phone to the other and have it work?
Also, I've heard that the SIM card also contains the contacts and what not, is this true? If it is, that would be nice to be able to switch phones with a SIM card and have all of your contacts already in there.
All of your account information is contained on the SIM, so, yes, you just move it to the new phone, and, as long as the phone is either locked to the same carrier or unlocked, it'll work. Simple as that.
Also, just for reference, GSM phones do not have an ESN. They have an IMEI, which is sort of like a serial number, but it isn't attached to your account the way an ESN is.
And to answer your last question, yes, you can store your contacts on the SIM or the phone. The advantage of using the SIM is portability, but the disadvantage is that you can only store one number per name. To have multiple numbers per name, plus e-mail addresses, physical addresses, etc., you have to store contacts in phone memory.
Thanks for the information. How does the IMEI work then?
The IMEI can be used for insurance purposes, verfiy the claim of ownership of said phone to receive an SIM unlock code or to prevent the phone from registering on to the network. In the last example US carriers don't currently ban lost or stolen phones from registering on any network, in Europe, yes they will ban lost or stolen phones from the network.
IMEI and SIM ICCD are transmitted to Cingular, so yes they do know what phone you're using even if it's a phone that they have never sold directly to you. Not that it matters since Cingular doesn't care what phones are used on their network, but they won't support anything else that's not offically registered with them.
That makes sense now, thanks. I'm looking forward to switching over to Cingular from Verizon. I believe I may be switching at a bad time though with all the new phones on the way out it seems almost pointless to switch now, get a phone, such as the 8125 or 2125, just to have the new ones come out and have to pay full retail price for.
One the other benefits, which I think you might have figured out already, is that you can buy any GSM phone (locked to Cingular SIMs, or Unlocked accepting any SIM) from Cingular, EBay, Costco, or any retailer, and use your SIM in it. Some people even pick up "pre-paid" phones from stores, and just put thier SIM in it (and not use the pre-paid.)
So you can have a collection of older phones and still use them whenever you want, unlike with Verizon or Sprint where once you switch, the older phones are pretty much paperweights.
I have a T-Mobile pre-paid SIM in addition to my normal Cingular SIM. I can take out the Cingular SIM and put in the T-Mobile pre-paid SIM to the same phone and now I'm using T-Mobile on the same phone. You can do this when traveling over seas as well assuming your phone supports thier frequenices.
I use T-Mobile SIM is really just a 'back up' in case I'm in T-Mobile's coverage and not in Cingular.
Unlocking is done by the carrier at their discression.
So if you bought a T-Mobile phone, locked. You'd have to call T-Mobile and ask them for the "subsidy unlock code" and they'll decide if they want to give it to you. Each carrier has a different policy on this. Cingular will (or used to) give you the unlock code if you owned the phone for more than 3 months. I don't know what T-Mobile's policy is.
Buying phones that are unlocked (or were never locked) are usually more expensive. It's called a subsidy lock code because it ensures a carrier that if they subsidize the cost of a phone to you, you can't just go use it on a competitor's network.
Some 3rd party services that you can find by searching online, will unlock some phones for a fee.
Also for a phone to work on Cingular, it should have the capability to do 850 and 1900 frequency bands. Some T-Mobile phones do only 1900.