Short answer: Credits flex to suit your needs.
More detail: Basically, a credit is a unit of cost used when sending or receiving text and voice messages. Credits work in two ways.
We have two options for our plans.
You can choose not to count incoming messages and have your bucket of credits reset at the end of each billing cycle (unused credits would expire and you would begin the next month with a fresh new bucket according to your plan). In our experience, incoming messages are usually 1-5% of your credit usage.
The other option is rollover credits. Incoming messages count out of your monthly bucket, but any unused credits rollover each billing cycle and never expire.
What we see is that for most churches, rollover credits are the better choice.
Unless you plan on asking for a lot of replies and interaction from your church, having your credits roll over each month generally outweighs the cost of paying for outgoing msgs/calls only.
Read more about how changing plans may affect your credits.