There is no expiration date on any funds you deposit that it has to be used up by. For example, if you were to deposit $10 (or any amount) today, you can keep that money in your account indefinitely... i.e. you could still have money left over from your $10 deposit in two years from now if you never finished using it up.
There is a requirement of using your phone at least *once every four months* in order to keep the account active and so that you shouldn't lose your phone number. That simply means you need to make one phone call for less than a minute every four months. Grand total cost? *Two Cents*. Or if you want to look at it from an annual basis, it will cost you *six cents* per year (coming out of your original balance) to keep your account, phone number and balance active indefinitely.
Which means it expires. Six cents or less it still costs six cents otherwise it expires either thru the six cents or thru the "non expiring service fee" if I don't make that phone call.
It doesn't expire would mean:
a. If I don't use the phone for 120 days (or whatever other number of days) they send me back a check refunding the money.
b. If I don't use the phone for 120 days (or whatever other number of days) they deactivate my number but allow me somehow to use that money as credit for a future account.
Now compared to others (so I assume from your post) the money expires and gets lost after x amount of days or whatever. So their policy is less of grab on the expiration but still expires.
My problem with "doesn't expire" is that its used as a selling point. No such thing, they cant keep accounting rolling over for ever, they HAVE to expire it for bookkeeping purposes.