By default Bing is limited to 2 minutes. Happily, that works well for voicemails. To transcribe all your recordings would be hugely expensive depending on the number of calls recorded and the average length of a call.
Further, voicemails are a single-person conversation. Adding multiple people into the transcription process would probably make the transcription quality suffer even more when multiple people are talking at the same time.
It's been some time since I looked at Bing for multi-party transcriptions of recordings that are greater than 2 minutes. I think it is possible to do, but it would require a 3rd party tool. Transcribing recordings is not currently an option with 3CX.
By the way, we use Bing too for our transcriptions in our 3CX Voicemail Manager add-on.