Telephony and voice APIs are one of the most active segments of the API and mashup world. For example, in the past week, two of the latest mashups in our directory use telephony APIs in innovative ways (the directory has 42 telephony-related APIs):
#blue is a web service to save text messages in an intelligent, searchable manner. It's a service of O2 UK, and sadly only currently available to O2 UK subscribers. However, it's a rather interesting web service, and it'd be great to see more service providers use a similar service. The fully functional #blue API provides a compliment to its web based service.