There are various sources of documentation on this site, including:

• API documentation for various languages
• Information about virtual phones for testing your applications
• Descriptions and detailed information for each of the Web21C services
• Pricing information
• DIY notes, for those who want to call the services without using the SDK
• How-Tos to help you use the SDK
• Papers presented at Conferences
• Videos demonstrating the services

If you can't find what you're looking for in the documentation or Frequently-asked Questions, you can always ask a question in the forums.