Telephony and voice APIs are one of the most active segments of the API and mashup world (our directory has 42 telephony-related APIs). For example, in the past week, two of the latest mashups in the directory use telephony APIs in innovative ways:
Ringful API Profile
Create rich voice and SMS mashups via simple RESTful API
Broadsoft Xtended API Profile
VoIP telephony services
Facebook API Profile
Social networking service
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Executive Editor, ProgrammableWeb. Author, Map Scripting 101. Lover, APIs.