Shameless plug: check out Tropo[1]: you can interface with SIP.

One thing to keep in mind regarding speech to text: if you need accuracy, it helps immensely if you can guide the recognizer with a list of expected answers. Un-hinted transcription will do its best to recognize words but without AI to understand the context, there are still quite a few possibilities for mis-recognition. More info about these two here[2].

[1] https://www.tropo.com

[2] https://www.tropo.com/docs/voice/transcription-vs-speech-rec...

(Full disclaimer: I'm in the Tropo BU)

I've never tried it myself as an api but a quick poke around the googles throws this out https://cloud.google.com/speech/

Amazon Lex? https://aws.amazon.com/lex/?nc2=h_l3_db

Shoot - I'd love an API you pass in txt in one language and get multiple languages back in txt and speech.