When a cell phone says that the "SIM card is full," this most likely this means that the SIM card has reached its maximum storage capacity. A user needs to delete contacts and text messages or upgrade to a larger SIM card for more storage space.
SIM, or Subscriber Identity Module, cards are portable memory chips that store a phone's information. This includes personal information, such as phone number, text messages and contact list. SIM cards need to be individually activated upon purchase, as they are tied to a cell phone account. SIM cards can also be used to conveniently transfer all of this information to a new SIM card-compatible phone.