You pay for outbound messages only. Standard text messages (SMS) are charged (1) message credit per person sent. Picture messages or messages above 160 characters (MMS) are charged (2) message credits. Call forwarding and voicemail are charged (1) message credit per minute.
Prices are subject to change at any time. Monthly credits expire monthly, and yearly credits expire yearly. If you purchase additional credits, those never expire (only if you cancel).
Please note: If you're on a shortcode, you'll pay for both incoming and outgoing messages.