I think you are slightly mistaken. Most all providers, including RIngCentral, do transcribe voice mail messages into a text document (or email). None that I am aware of transcribe entire call recordings into text documents. Some calls can be quite long, not to mention that a transcriber (human or digital) would not know who the participants of a conversation are and some voices might sound similar. This would make a call recording very hard to transcribe.
RingCentral does offer the voice mail transcription in the Premium or Ultimate editions of their Office Plan.