When you create a NumberProxy you have several options based on how long you need the number and how much you plan to use it. Each plan has a different configuration of duration, minutes, and messages. For example, a Short NumberProxy is good for 7 days, 20 minutes of voice, or 60 text messages.
What this means is that a Short NumberProxy will expire at the earliest of the following events:
Unused minutes, messages, and duration are forfeited when the number is tossed. For example, leftover voice minutes at the end of a 30-day plan cannot be recovered if it expires.
Numbers can be recharged via the button on the Numbers page. Any additional duration or minutes purchased as an extension are additive to your current duration and minutes. (For example, if you buy a 30-day recharge for an existing number with 10 days left on it, it will result in a number that has 40 days left to go.)
When you manually toss a number, any remaining duration, minutes, or messages are forfeit.