Open the web console of 3CX then on the left under General is System Prompts.
You will see quite a few of them. Might not be all the prompts you can get on the system but looks like it covers all the VM stuff.
It will show things like:
"And" in the phrase "You have two messages: two heard and one unheard". Informational intonation.
This will give you what the phrase is and what the use is. Record the new one and then click "Browse" to find the new wav file. It will upload the new one and put it to use.
And stealing from the screen:
"For best performance, it is best to save the prompt in PCM, 8 kHz, 16 bit, Mono format"