your concept using Wait() won’t work here.
Try it like this:
exten => s,1,Dial(DAHDI/1234,30) ; This will ring the phone 30s
exten => s,n,BackGround(wellcome-message)
exten => s,n,Voicemail(1234)
exten => #,1,Hangup()
So, of you answer the call within 30s, you’ll get the call on your
phone. After 30s, the Voicemail will answer the phone.