Voicemail drop is the outbound pattern where an agent (or automated dialer) places a call, detects the answering machine, plays a pre-recorded message, and hangs up. The recipient gets a voicemail; the agent never speaks live, saving time on cold outbound where most calls go to voicemail anyway.
\nOften conflated with ringless voicemail (RVM) — which is a different and far more legally treacherous technique. See the section below.
\nplay.client.calls.create(\n to=\"+13105551234\",\n from_=\"+12125551111\",\n url=\"https://your.api/voicemail-drop.xml\",\n machine_detection=\"DetectMessageEnd\",\n machine_detection_timeout=10,\n machine_detection_speech_threshold=2400,\n machine_detection_speech_end_threshold=1200,\n machine_detection_silence_timeout=5000,\n)\nmachine_detection=\"DetectMessageEnd\" tells SignalWire to wait through the voicemail greeting and only fire the callback after the beep — so the recording starts in the actual message slot, not over the outgoing greeting.
The webhook URL receives a AnsweredBy parameter:
| AnsweredBy value | Meaning |\n|---|---|\n| human | A person answered |\n| machine_start | Voicemail/IVR started (early detection) |\n| machine_end_beep | Voicemail beep detected, message slot open |\n| machine_end_silence | Machine ended without a clear beep |\n| machine_end_other | Machine ended via tone or hang-up |\n| fax | Fax tone detected |\n| unknown | Detection inconclusive |
<Response>\n <Play>https://cdn.example.com/voicemail-drop-v3.mp3</Play>\n <Hangup />\n</Response>\nversion: 1.0.0\nsections:\n main:\n - answer:\n answer_on: machine_end_beep\n - play:\n url: https://cdn.example.com/voicemail-drop-v3.mp3\n - hangup: {}\n| Parameter | Default | Description |\n|---|---|---|\n| machine_detection_timeout | 30 sec | Max time to make a detection decision |\n| machine_detection_speech_threshold | 2400 ms | Min duration of initial speech to consider it a machine greeting |\n| machine_detection_speech_end_threshold | 1200 ms | Silence after greeting to call it \"ended\" |\n| machine_detection_silence_timeout | 5000 ms | Silence period that indicates a human pause vs. machine end |
Tune for your audience: business phones with long voicemail greetings need longer thresholds; personal mobile voicemails are shorter.
\nAMD is not perfect. Industry-typical accuracy: 90-95% on human/machine distinction. Failure modes:
\n| Failure | Consequence |\n|---|---|\n| Human classified as machine | Pre-recorded plays over a live person's \"hello\" — embarrassing, annoying, possibly TCPA violation |\n| Machine classified as human | Voicemail drop didn't fire, no message left |\n| Beep missed | Message plays mid-greeting, partially recorded |
\nProduction callbacks should handle the ambiguity:
\nif answered_by == \"human\":\n return live_message_response() # different copy, or hand to agent\nelif answered_by in (\"machine_end_beep\", \"machine_end_silence\"):\n return voicemail_drop_response()\nelif answered_by == \"machine_start\":\n # too early — voicemail greeting still playing\n return hold_response() # wait, or call back\nelse:\n return hangup_response() # don't risk it\nCritical distinction. Both deliver pre-recorded messages but the mechanism is different.
\n| Attribute | Voicemail drop | Ringless voicemail (RVM) |\n|---|---|---|\n| Mechanism | Standard outbound call, AMD-triggered drop | Direct insertion to voicemail server, bypassing the phone ring |\n| Recipient experience | Phone rings, may show missed call | No ring, voicemail appears |\n| Carrier interaction | Standard PSTN call setup | Server-side voicemail injection (often deemed not a \"call\" by vendors) |\n| TCPA status | Treated as an outbound call → subject to TCPA full | Vendor argument: not a call, not subject to TCPA. FCC and most courts disagree. |\n| Risk | Manageable with consent | High legal risk |
\nSignalWire does not offer RVM as a product. Voicemail drop via AMD is the supported, legally safer path.
\nThe 1991 Telephone Consumer Protection Act regulates auto-dialed and pre-recorded calls. Voicemail drop is unambiguously a pre-recorded call.
\n| Call type | Required consent |\n|---|---|\n| Pure transactional (appointment reminder, delivery notification) | Implied or express consent (relationship establishes it) |\n| Service notification (account changes, security alerts) | Express consent (opted into the relationship) |\n| Telemarketing / promotional | Prior express written consent (PEWC) — signed disclosure with TCPA-compliant language |
\nPEWC is the strict standard. Requires:
\nTCPA private right of action: $500-$1500 per violation. Class actions over voicemail drop campaigns regularly settle for tens of millions.
\nFederal rule: telemarketing calls only between 8 AM and 9 PM local time of the called party. Voicemail drop counts.
\nTelemarketing voicemail drops to numbers on the National DNC list = $1,500/violation. Scrub the call list against DNC weekly minimum, ideally daily.
\n| State | Additional rule |\n|---|---|\n| California | Mini-TCPA (CIPA) — adds wiretap-style claims for unconsented recordings |\n| Florida | FTSA — explicit written consent for autodialed calls; stricter than TCPA |\n| Washington | Mini-TCPA — robocall restrictions |\n| Oklahoma | TCPA OK — similar to Florida |
\nA Florida FTSA voicemail drop without compliant consent is $500 per call minimum, with class action exposure.
\nAppointment reminders, delivery confirmations, account alerts.
\nLive agent dials. If human answers, agent talks. If machine answers, agent triggers the drop button — pre-recorded plays, agent moves to next call.
\ndef agent_drop_voicemail(call_sid, message_url):\n client.calls(call_sid).update(\n twiml=f\"\"\"\n <Response>\n <Play>{message_url}</Play>\n <Hangup />\n </Response>\n \"\"\"\n )\nThis is the most defensible pattern because a human is attempting live conversation first — the drop is just a polite \"leave a message\" move.
\nDialer auto-runs through a list, AMD-triggers drop on every voicemail. Highest TCPA risk. Only acceptable with iron-clad PEWC, DNC scrubbing, and recording.
\nEven with consent, carriers may filter:
\nMitigation: see STIR/SHAKEN, use multiple numbers, pace the dial rate (5-15 calls per minute per number).
\nRecorded message best practices:
\nRequired opt-out language (TCPA 47 CFR 64.1200(a)(7)):
\n\n\n\"If you do not wish to receive future calls from us, you can request to be added to our internal do-not-call list by pressing 9 now, or by calling [number].\"
\n
For pre-recorded marketing calls, the opt-out mechanism must be activated within 2 seconds of the message ending. Press-9 to opt out is the standard implementation.
\nDetectMessageEnd not Enable for AMD mode.