While it is possible to send a notification of a voicemail, but a text is limited to alphanumeric characters you can't attach a file.
Texting, in itself , requires either a paid service/subscription, or an email to text service from by the destination phone provider.
This is one of many sites with a list of various mobile providers email to text information.
I think Leejor misunderstood the question. It seems you would like to transcribe a voicemail into text and send the text to the email recipient, along with the wav file.
One way is to use a Google Voice account (free) and have GV be the voicemail service for your user instead of 3CX. I've been doing it for years, I will add that the best way to describe the transcriptions is with the word 'hysterical'.