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What Is SIP Telephony: A Beginner’s Guide

SIP Trunks & PBX — Scalable, Secure Voice Infrastructure for Modern Business

Cut telephony costs, improve call quality, and scale instantly. Our SIP trunks and PBX solutions deliver secure global calling, advanced contact-center features, and full control over your voice infrastructure — whether you prefer cloud, on-prem, or hybrid deployment.

What Is SIP

What Is SIP Trunks

SIP trunk

What is a PBX

SIP Software

SIP Providers

On-Premise PBX

Hosted (Cloud) PBX

Hybrid PBX

Lower costs: Replace expensive PSTN lines and per-minute international premiums with competitive SIP termination and flexible pricing.
Scalability: Add channels instantly to handle seasonal spikes or marketing campaigns.
Global reach: Local DIDs and A-Z international termination for global operations.
Advanced features: IVR, call recording, whisper/barge, skill-based routing, CRM integrations, and analytics.
Business continuity: Automatic failover, geo-redundancy, and disaster recovery options.
Security & control: TLS/SRTP, SBCs, IP ACLs, and fraud monitoring for sensitive operations (e.g., finance, gaming, healthcare).

Core Benefits — Why Your Business Needs SIP Trunks + PBX

Caller dials a number → network routes call to SIP Provider.
SIP Provider forwards call to your SIP trunk.
Your PBX receives the SIP INVITE and routes to an extension, queue, or IVR.
Media (RTP) flows directly between endpoints or via media proxies/SBCs for security.

Simple flow:

A PBX (Private Branch Exchange) is the internal phone switching system for an organization. It manages extensions, call routing, call queuing, IVR, voicemail, conferencing, and integrations with CRM or billing systems.

What is a PBX?

Replaces traditional ISDN/PRI lines with IP channels.
Supports multiple concurrent calls per trunk (channels scale by configuration).
Enables international termination (A–Z routes), DID provisioning, and number portability.
Works with SIP-compatible PBX, softswitches, cloud contact centers, and SBCs.

Key characteristics:

A SIP trunk (Session Initiation Protocol trunk) is a virtual telephone line that carries voice, video, and messaging over an IP network. Instead of physical PSTN copper lines, a SIP trunk connects your telephony system (PBX or contact center) to the public telephone network and other SIP endpoints using the internet or private IP links.

What is a SIP Trunk?

2) Hosted (Cloud) PBX + SIP Trunk

Use when: Compliance, low-latency LAN requirements, existing telephony investment.
Notes: Deploy an SBC for SIP normalization, security, and NAT traversal.

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Call Center

|
Ext2

/
Ext1

PSTN / SIP Provider
|
Internet or MPLS
|
[SBC]
|
On-Prem PBX

1) On-Prem PBX + SIP Trunk (Direct)

Below are common connection patterns. Use these diagrams as blueprints when planning deployment.

Connection Schemes (Architectural Diagrams & Explanations)

3) Hybrid PBX (On-Prem Core + Cloud Burst)

Use when: Fast deployment, remote teams, limited on-site IT.
Notes: Provider handles SBC, media proxies, and DID provisioning.

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Agents

|
Mobile

/
WebRTC

SIP Provider (termination)
|
Internet
|
Cloud PBX (Provider)

4) SIP Trunk to Cloud Contact Center (API-driven)

Use when: Need for local control + cloud elasticity.
Notes: Best for redundancy and peak traffic handling.

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Local Routing

|
CRM

/
Agents

Cloud SIP Hub (SBC/Controller)
|
On-Prem PBX (Core)

SIP Provider A

|
Internet
\

SIP Provider B
(backup)
|
Internet
/

5) Disaster Recovery / Failover Scenario

Use when: Outsourced contact center operations, integrations with marketing stacks.

SIP Trunk to Cloud Contact Center (API-driven)

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Dialer Analytics

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Predictive Dialer

/
IVR

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CRM / Support Portal (integrations via API)

SIP Provider
|
Internet
|
Cloud Contact Center (SaaS)

Use when: Regulatory or SLA requirements demand instant failover.
Notes: Configure DNS SRV, SIP forking, or routing at carrier level.

-> Secondary SIP Trunk -> Secondary PBX (Region B)

Primary SIP Trunk -> Primary PBX (Region A)
\

Pay-As-You-Go: Per-minute termination + per-DID/month. Low commitment, good for testing.
Committed Capacity: Monthly commitment for a block of minutes or channels, lower per-minute rates.
Dedicated Circuits / MPLS: For large enterprises requiring guaranteed bandwidth and QoS.
SaaS PBX Subscription: Per-user per-month with included minutes or separate termination billing.

Pricing Models (Typical)

SBC (Session Border Controller): Mandatory for enterprise deployments — provides topology hiding, SIP normalization, policy enforcement, and media anchoring.
Encryption: Use TLS for SIP signaling and SRTP for media where supported.
Authentication & ACLs: IP whitelisting, username/password, and certificate-based auth.
Fraud Prevention: Real-time monitoring, anomaly detection (spikes in concurrent calls), and rate limiting.
Regulatory Compliance: E911, CDR retention policies, call recording consent (GDPR/region-specific), and number portability rules.

Security & Compliance Essentials

Traffic audit: Measure current channels, peak concurrency, and average call durations.
Quality baseline: Test existing carriers for latency, jitter, packet loss.
Network readiness: Ensure QoS, VLANs for voice, sufficient bandwidth.
SIP credentials & routing plans: Gather IPs, credentials, and preferred route priorities.
SBC deployment/placement: Decide on on-prem vs cloud-managed SBC.
Number porting roadmap: Collect LOAs and timelines for DID porting.
Failover & DR: Configure secondary trunks and test failover.
Monitoring & alerting: Set up CDRs, QoS dashboards, and fraud alerts.
STAFF TRAINING: Admins and contact center agents trained on PBX console and softphone apps.
Go-live & phased cutover: Pilot, monitor metrics, then full migration.

Migration & Onboarding Checklist

About: International online casino with KYC, VIP managers, and outbound reactivation campaigns.
Problem: Frequent blocking in some GEOs, unstable CLI delivery, and need for high ASR for verification calls.
Task: Ensure stable CLI delivery, reduce blocked calls, and integrate PBX with CRM for verification flows.
Solution: Provisioned specialized SIP routes optimized for gaming traffic, deployed a hybrid PBX (on-prem core + cloud burst), implemented fraud controls and SIP auth, and created automated verify-callback flows from the PBX integrated with CRM.
Result: ASR rose from 46% → 82%, blocked calls reduced by 68%, verification times halved, and deposit conversions improved significantly.

Case C — iGaming Operator (High-Risk Traffic; PBX + Specialized SIP Routes)

About: SaaS provider with 300 remote employees across 15 countries providing 24/7 support.
Problem: Fragmented softphone solutions, inconsistent call quality, and complex per-country phone provisioning.
Task: Centralize telephony, provide stable softphones, and provision local DIDs for regional teams.
Solution: Migrated to hosted PBX with integrated SIP trunks for local DID provisioning. Rolled out managed softphone apps and centralized call analytics.
Result: Deployment in 4 weeks, reduced onboarding time for new hires by 60%, and a 30% improvement in first-call resolution due to better routing and unified CRM integration.

Case B — Global SaaS Company (Hosted PBX + SIP Trunk)

About: Retail chain with 120 stores, centralized customer service, and POS support lines.
Problem: High PSTN costs and difficulty scaling voice channels during holiday peaks. Existing on-prem PBX lacked flexible international calling.
Task: Reduce telecom spend, add burst capacity, maintain local numbers for stores.
Solution: Deployed SIP trunks with burstable channels, integrated with the existing on-prem PBX via an SBC. Implemented LCR for international calls and DID mapping per store.
Result: Telecom costs dropped by 45%, peak-hour call blocking eliminated, and stores retained local numbers — improving customer satisfaction and lowering monthly costs.

Case A — Regional Retail Chain (On-Prem PBX + SIP Trunk)

Three Realistic Case Studies

Learn more about SIP Trunk solutions →

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Frequently Asked Questions (FAQ)

question

How many concurrent calls does one SIP trunk support?

answer

A SIP trunk can support multiple concurrent sessions; capacity is defined by channels or by the provider’s configuration. You size trunks by peak concurrent calls (CC) + burst margin.

question

Do we need new hardware to use SIP trunks?

answer

Not always. Cloud PBX needs only internet and softphones. On-prem PBX requires an SBC or compatible gateway. We’ll audit and recommend minimal changes.

question

Can we keep our existing phone numbers?

answer

Yes — number porting (DID porting) is supported in most regions. Porting timelines vary by country.

question

How do you prevent toll fraud?

answer

We enforce IP whitelists, strong authentication, rate limits, real-time fraud detection, and configurable call routing policies.

question

Is voice encrypted?

answer

Yes — where supported we use TLS for SIP signaling and SRTP for media. Some legacy endpoints may not support SRTP; we recommend an SBC to mediate and enforce encryption.