When you activate a prepaid subscriber identity module, or "SIM," card for a phone, the call, text and Internet credit you purchase expires after a certain period of time, as specified by the mobile carrier. The SIM card itself never expires, as the SIM serves only to allow the handset to identify the cellular network.
The date at which service credit on a SIM card expires depends on the carrier, but is usually a fixed period -- six months or a year, for example -- after you load the credit that has most recently been loaded on the card. If you've added credit recently, that credit won't expire until a later date, although any credit you added before then that still remains once it reaches its expiration date will expire on schedule.
You have several options for checking both the amount of credit you have on your card and the date you can expect it to expire. The simplest way is to visit a retail store or call the customer service phone number for your cellphone carrier, although you may be able to see this information within the interface you use to manage your account online, if your carrier offers this service.
Why Prepaid Credit Expires
When you purchased prepaid credit, you pay a rate commensurate with the prices the cellphone carrier offers at that point in time, a price the cellphone carrier is willing to offer for its specified term. If prepaid credit never expired, customers could purchase large amounts of credit when rates are cheap and horde it far into the future, even if costs go up dramatically.
Although cellphones from a variety of carriers accept SIM cards, the SIM card you use is always tied to the carrier from whom you purchased it, something that may become problematic if you want to use a SIM card at a later date. For example, if you have prepaid credit left over on a T-Mobile SIM card and purchase an AT&T handset, you aren't able to use the credit on the new device unless you "unlock" the phone, an unauthorized procedure that risks damaging it.