extensions.conf

exten => 3XXXXXXX,1,Wait(20)
exten => 3XXXXXXX,n,Voicemail(u1234)
exten => 3XXXXXXX,n,Hangup

Debug output from capi

    -- ISDN1: info element Sending Complete
    -- Executing Wait("CAPI/ISDN1/3XXXXXXXX-3", "20") in new stack
DISCONNECT_IND ID=001 #0x6b2c LEN=0014
  Controller/PLCI/NCCI            = 0x201
  Reason                          = 0x0

DISCONNECT_RESP ID=001 #0x6b2c LEN=0012
  Controller/PLCI/NCCI            = 0x201

  == Spawn extension (isdn-in, 3XXXXXXXXX, 1) exited non-zero on 'CAPI/ISDN1/3XXXXXXXX-3'
  == ISDN1: CAPI Hangingup
  == ISDN1: Interface cleanup PLCI=0x201