You can implement voicemail detection using a custom dialplan in Asterisk with the AMD() application.

Important:

There is no universal configuration. AMD parameters depend heavily on:

• Country
• Landline and Mobile Carriers
• Audio quality
• Language and voicemail behavior

Best practice:

• Start with standard parameters
• Record and analyze real calls
• Tune based on your own environment

Example Dialplan (basic AMD implementation):

;; START AMD_CHECKER
   ;same => n,AMD(${INITIALSILENCE}, ${GREETING}, ${AFTERGREETINGSILENCE}, ${TOTALANALYSIS}, ${MINIUMWORDLENGTH}, ${BETWEENWORDSILENCE}, ${MAXIMUMNUMBEROFWORDS}, ${SILENCETHRESHOLD}, ${MAXIMUMWORDLENGTH})
   same => n,AMD(2500, 1500, 800, 5000, 100, 50, 4, 256, 1500)
   same => n,NoOp(AMD STATUS ${AMDSTATUS})
   same => n,ExecIf($["${AMDSTATUS}"="MACHINE"]?WaitForSilence(2200,1,25))
   same => n,NoOp(WAITSTATUS ${WAITSTATUS})
   same => n,ExecIf($["${AMDSTATUS}"="MACHINE"]?Background(custom/fAudioVoicemailMessage))
   same => n,ExecIf($["${AMDSTATUS}"="MACHINE"]?UserEvent(ATTRIBUTE,Uniqueid:${UNIQUEID},APPT:VOICEMAIL))
   same => n,ExecIf($["${AMDSTATUS}"="MACHINE"]?UserEvent(CALLSTATUS,Uniqueid:${UNIQUEID},V:VOICEMAIL)
   same => n,ExecIf($["${AMDSTATUS}"="MACHINE"]?Hangup())
;;; END AMD CHECKER