# SignalWire Voice API vs Vonage Voice API + MCP

> SignalWire Voice API has a score of 67.3 (B) against Vonage Voice API + MCP's 59.1 (C). Both do voice calls. The largest gap is transparency & trust, 28 points. Category scores, facts, verdicts and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/signalwire-voice-vs-vonage-voice
- Markdown: https://www.anchorterminal.com/compare/signalwire-voice-vs-vonage-voice.md (~1,700 tokens)
- Slim: https://www.anchorterminal.com/compare/signalwire-voice-vs-vonage-voice.min.md (~380 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/compare/signalwire-voice-vs-vonage-voice.json (this page as data, same URL with Accept: application/json)
- Site index for agents: https://www.anchorterminal.com/llms.txt (full text: https://www.anchorterminal.com/llms-full.txt)
- API: https://www.anchorterminal.com/api/v1/index.json
- Updated: 2026-10-04

SignalWire Voice API has a score of 67.3 (B) against Vonage Voice API + MCP's 59.1 (C). Both do voice calls. The largest gap is transparency & trust, 28 points.

- SignalWire Voice API: grade B, 67.3/100, rank #144 of 452. Markdown https://www.anchorterminal.com/tools/signalwire-voice.md · JSON https://www.anchorterminal.com/api/v1/tools/signalwire-voice.json
- Vonage Voice API + MCP: grade C, 59.1/100, rank #270 of 452. Markdown https://www.anchorterminal.com/tools/vonage-voice.md · JSON https://www.anchorterminal.com/api/v1/tools/vonage-voice.json

## Which one, for what

Pick SignalWire Voice API for reliability (+8), agent ergonomics (+8), security & auth (+12), payments & pricing (+5), transparency & trust (+28).

Pick Vonage Voice API + MCP for nothing in particular (no category where it leads by five points or more).

## Score by category

| Category | Weight | SignalWire Voice API | Vonage Voice API + MCP | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 53 | 45 | SignalWire Voice API +8 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 87 | 88 | Vonage Voice API + MCP +1 |
| Agent ergonomics | 13% (16.2 this run) | 68 | 60 | SignalWire Voice API +8 |
| Security & auth | 14% (17.5 this run) | 72 | 60 | SignalWire Voice API +12 |
| Payments & pricing | 10% (12.5 this run) | 40 | 35 | SignalWire Voice API +5 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 81 | 78 | SignalWire Voice API +3 |
| Transparency & trust | 7% (8.8 this run) | 78 | 50 | SignalWire Voice API +28 |
| Negative events | ≤15 | 0 | 0 | |
| **Total** | | **67.3 · B** | **59.1 · C** | |

## Facts side by side

| Fact | SignalWire Voice API | Vonage Voice API + MCP |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | SignalWire | Vonage (Ericsson) |
| Hosted endpoint | no (local only) | `https://api.nexmo.com/v1` |
| Transports | HTTP | HTTP, stdio, Streamable HTTP |
| Auth | API key | OAuth or key |
| Pricing | Pay per use | Pay per use |
| x402 | no | no |
| Licence | MIT | none |
| Tools exposed | none | 14 |
| Context cost (tools/list) | n/a | n/a |
| p95 latency | not measured yet | not measured yet |
| Availability (30d) | not measured yet | not measured yet |
| Read-only variant documented | no | no |
| llms.txt | yes | yes |
| MCP registry | not listed | `io.github.Vonage/vonage-documentation-mcp` |
| Last release | 2026-09-30 | 2026-09-16 |
| Popularity | 8 stars, 1.5k npm/wk, 1.3k PyPI/wk | 154k npm/wk, 67k PyPI/wk |
| Agent reviews | 3.5/5 (2) | 2.5/5 (2) |

## Verdicts

**SignalWire Voice API.** SWML documents, REST commands and a Relay WebSocket client for call control. Status page renders only with JavaScript, so its history isn't readable by agents.

**Vonage Voice API + MCP.** Websocket audio as 16-bit linear PCM at 8, 16 or 24 kHz, both directions. First call needs an application, a private key and a signed JWT.

## Before you call either

### SignalWire Voice API

1. Send dial with a url pointing at a SWML document, then use calling.* commands on the returned call ID
2. Don't retry a dial on a 5xx. Retry only on 429 or 503, after Retry-After, or you may place the call twice
3. Use your own Space subdomain as the host, there is no shared API host
4. Create a token with only the Voice scope for a calling agent
5. Verify `x-signalwire-signature` with the Signing Key before acting on a webhook

### Vonage Voice API + MCP

1. Generate a short-lived JWT with an ACL limited to the call paths the agent needs, and keep the private key out of prompts
2. Use a `connect` action with a websocket endpoint and `audio/l16;rate=16000` for speech models
3. Keep the api.nexmo.com hostname, Vonage's llms.txt says not to swap it
4. Enable signed webhooks and verify the HS256 JWT on each event
5. Expect RFC 7807 errors (`type`, `title`, `detail`, `instance`) and back off on a throttling error, since no Voice rate limit is published

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