Use custom audio in Voicemail blast

[DMNyaku (Douglas Mandela)]

I have PBX Version:16.0.21.9 running Asterisk Version:16.24.0.
I have set up an IVR accessible from PSTN dial, then routing the call to the relevant queue based on user selected options. Everything as far as IVR and Queues is working fine.
My issue is on blasting voicemail, when no agent is available in a queue. I want to use a custom audio file rather than the “Read Group Number and Beep Only” options available.
I have tried to come up with vmblast-grp-custom & app-vmblast-custom contexts in extensions_custom.conf file but so far I haven’t managed, any help with this?

[DMNyaku (Douglas Mandela)]

This here seemed to sort it out,
[from-internal]
exten => Queue,1,NoOp(HERE)
same => n,Goto(vmblast,VMBlastNumber,1)

[vmblast]
exten => VMBlastNumber,1,NoOp(VOICEMAIL BLAST)
same => n,Playback(vm-intro)
;same => n,Playback(beep)
same => n,Goto(vmblast-grp,VMBlastNumber,1)
;same => n,Hangup()