Swapping SIM cards and carriers

So I'm changing my carrier to T-Mobile (I currently have AT&T) because they have the best coverage where I live. However, I'm not sure what data I will lose when I swap the cards. Will I lose all of my data like apps, wallpapers, settings etc. or will I just lose things like phone numbers and contacts?

I speculate - I'm assuming - only data pertaining to your subscription with AT&T will be affected. For example: wallpapers, apps, and contacts will be fine. My reasoning is because I don't think a SIM card could hold a gigabyte of information, and also SIM means "Subscriber Identity Module" - module that has subscriber identity.

Edit: Did some reading, apparently SIM cards hold contact information and saved text messages.

The SIM card holds details on your phone number and your contacts if you have saved them to it. Most newer phones save to the phone by default, or at least that's what my 2 newest phones have done.

Connect your phone to a PC using USB or Blutooth(even my ancient Nokia 6310i had this feature) and back up your stuff. How you do this depends on what model your phone is.

SIM cards can hold a limited amount of information, in practice messages and contacts tend to be stored on it. (Your phone can also have an SD card as well as built in memory).

This data can be backed up easily... in fact most phones support copying this data to their own memory/SD card. (I could do this on my *ancient* Nokia 2210, the brick-phone one could use as an improvised bludgeoning device).

I can't tell you anything more concrete without knowing what kind of phone you have.

Tch...well that was not smart. I did not realize that I had forgotten my phone model. It is a Motorola Atrix 4G.

Anyways, thank you everyone. This confirmed my previous belief and the fact that my mother was completely wrong. I just have to write down / move my phone numbers then since I have never had text messaging therefore I do not need to worry about any lost messages.

I think mine only holds the number and what ever 4g needs to run. Verizon didn't use them until they got smart phones. I never had a sim card phone until now. Anyways my contacts are transfer over online backup and my sd card back up. I should test if i can call without the sim card but it's a pain to break down my otter shell and have to clean and mount the screen protector.

before you change it make an backup of the system and the microsd(hc/xc{yes it exists})
then change your sim-card and see what data is lost

if none of your data is lost then you go on with the new provider
if some data is lost from a game , this could happen
if some data of the system is lost try the back-up
if the data is lost after this ask your new provider for help
if the data cant be recieved or be restored try an new phone, you have cheap smartphones for 100$/€
of course they arent as good as your old one but still good to go

if you are going to take a iphone 5/other

you will have to transform your sim to micro sim

!!!use a micro-sim cutter for it!!!!
if you do it with an scissor you will make a mistake and it wouldnt fit anymore

(cases to transform the micro-sim to sim are delivered with the cutter)