Here are the top voice call API providers worth considering, based on reliability, global reach, and developer experience:

1. Twilio
Strengths:

Extensive global infrastructure.

Rock-solid uptime.

Great SDKs and documentation.

Use case fit: Ideal for scaling from MVP to enterprise-grade systems.

Pricing: Pay-as-you-go; can get costly at scale.

Extras: Built-in call recording, conferencing, and AI features (like real-time transcription).

2. Vonage (formerly Nexmo)
Strengths:

Strong voice quality in multiple regions.

Flexible call control with WebSockets.

Good developer portal.

Use case fit: Great for support systems with a need for IVR or call routing logic.

Pricing: Slightly lower than Twilio in some regions.

3. Plivo
Strengths:

More affordable than Twilio.

Clear pricing and predictable costs.

Global coverage with high delivery rates.

Use case fit: Mid-sized projects that want solid features without the Twilio cost.

Watch out for: Slightly less intuitive dashboard than Twilio or Vonage.

4. MirrorFly (Self-hosted and SaaS)
Strengths:

Offers both on-premise and cloud options.

Useful if you need full control over infrastructure (compliance-heavy use cases).

Use case fit: Internal call systems or apps needing on-premise deployment.

Caveat: Longer setup time compared to plug-and-play APIs like Twilio.

5. SignalWire
Strengths:

Built by the creators of FreeSWITCH.

Low-latency calls with flexible call flows.

Use case fit: Projects needing custom call logic and real-time media handling.

Bonus: Good for developers who want more programmatic control.

6. Agora
Strengths:

Real-time engagement APIs for voice, video, and streaming.

Global edge network for low latency.

Use case fit: Interactive support with in-app voice.

Drawback: Less focused on traditional PSTN call flows.

My Recommendation
If you're building a customer support system with global reach, start with:

Twilio for fastest go-to-market and broadest toolset.

Plivo if you're budget-conscious and need PSTN and SIP coverage.

Vonage if you're planning to integrate voice with SMS or WhatsApp too.

Want to self-host or have strict data control? Look deeper into MirrorFly or SignalWire.