Voice limitations and edge cases
Before you architect your Twilio Voice application, review these limitations and edge cases. These constraints may influence your design decisions, capacity planning, and feature selection.
See Related reference documentation to learn more about the API calls and TwiML used in this guide.
| Area | Limit | Notes |
|---|---|---|
| Outbound calls per second | 1 CPS (default) | Increase from Twilio Console |
| Call duration | 4 hours | Call terminates automatically |
| Recording duration | 4 hours | Per call |
| Transcription length | 2 minutes | Maximum audio length |
| TwiML document size | 64 KB | Notification sent when exceeded |
| RTP timeout | 60 seconds | 62 seconds on SIP public edges |
| Conference participants | 250 | — |
| Queue participants | 100 default, 5,000 max | Calls rejected when max reached |
| SIP domains per account | 100 | — |
| Registrations per AOR | 10 | 11th registration rejected |
| Credential lists per SIP domain | 100 | — |
| Users per credential list | 1,000 | — |
| Registration expiration | 600–3,600 seconds | 10 min to 1 hour |
| IPs per access control list | 100 | Wildcards not supported |
| Media Stream tracks (unidirectional) | 4 per call | DTMF not supported |
| Bidirectional streams | 1 per call | Inbound track only |
| CNAM name length | 15 characters | Must begin with a letter |
If the moderator disconnects, the conference automatically ends for all participants. Design your application with moderator failover in mind.
AMD doesn't always return the correct answer. Tuning parameters involves tradeoffs:
- Low
MachineDetectionTimeout: moreunknownresults. - Low
MachineDetectionSpeechThreshold: more false machine detections for short human responses. - High
MachineDetectionSpeechEndThreshold: delays human detection and can misinterpret human silences as machines.
AMD performance also depends on your webhook response time. Host media in the region closest to your Twilio region (us-east-1, eu-west-1, or ap-southeast-2).
- Geographic restriction: Available only for US phone numbers. Canadian and US/Canadian toll-free numbers cannot be assigned to CNAM profiles.
- Carrier-dependent display: CNAM displays only if the recipient's carrier supports it (enabled by default on landlines, opt-in on most mobile carriers).
- Propagation time: Takes 48–72 hours to propagate to all US carriers after approval.
- Regional behavior: Registered endpoints are region-specific. If you don't specify an edge location, it defaults to Ashburn, VA.
- Parallel ring: All endpoints registered to the same AOR across all edge locations ring simultaneously. Factor this into your call flow design.
- Secure media: When enabled, Twilio rejects calls that don't negotiate SRTP. Configure your clients for SRTP before enabling.
- Routing: Emergency calls are routed only when numbers enabled for emergency calling are used as the Caller ID. Calls from other numbers are rejected.
- Webhook bypass: Twilio ignores your webhook configuration and routes directly to the PSAP.
- Fees: $75 per 911 call if the phone number lacks a registered emergency address. $0.75/month per number for emergency address association.
- Encryption availability: Available only to Enterprise Edition and Security Edition customers.
- Encryption irreversibility: Once enabled, no one at Twilio (including support) can decrypt recordings. Only the account holder with the private key can.
- External storage: When external S3 storage is enabled, Twilio does not retain copies in Twilio cloud storage.
- Bidirectional DTMF: Supported only in the inbound direction (Twilio to media server). Outbound DTMF is not supported.
- Audio format: Media must be encoded as
audio/x-mulawat 8,000 Hz in base64. Payloads must not contain audio file type header bytes.
- Static IP regional limitation: Not available for Ireland (IE1) or Australia (AU1). Only supported with US1.
- Static IP availability: Limited; requires Twilio Interconnect and contact with Sales.
- Origination authentication: Twilio does not support credential-based auth with carriers for origination calls. Your carrier must allow traffic from regional Twilio IP addresses.
- Secure media scope: The TLS/SRTP setting only affects termination (inbound) traffic, not origination.
All outgoing calls to the US and Canada are blocked if they use an invalid, unassigned, or do-not-originate (DNO) calling number. Calls forwarded through Twilio are also blocked. Calls with the ANONYMOUS From parameter are not blocked.