The number emails you can send from the system is unlimited, and the of text/voice messages is based on a credit system. 1 credit = 2 text messages or 1 ringless voicemail.
For example, if you have a plan of 1,000 credits, you will be able to send any combination of 2,000 text messages or 1,000 voicemails per month.
If you are uploading a large list of contacts or are concerned about overages, you can buy more credits. These credits never expire and always roll-over to the next month.