SIP Trunking for ISPs Offering Voice Services

Internet service providers (ISPs) that already have network infrastructure and relationships with business customers are in a strong position to offer SIP trunking services. The challenge, however, goes beyond connectivity: you need the right platform for call routing, border security, billing, and provisioning. The SipPulse platform was designed exactly for this scenario, offering an integrated ecosystem that covers every layer of a voice operation.

What Is SIP Trunking

SIP trunking is the delivery of voice channels via SIP protocol over the IP network, replacing legacy E1/T1 trunks or analog lines. The business customer connects their IP PBX or contact center to the ISP's SBC, which in turn interconnects with the public telephone network (PSTN) or with other operators via SIP.

For the ISP, this represents a recurring revenue opportunity with attractive margins, provided the voice infrastructure is properly sized and automated.

The SipPulse Platform: Everything an ISP Needs

SipPulse offers three products that, together, form the backbone of a professional SIP trunking operation.

SipPulse SoftSwitch: The Brain of the Operation

The SipPulse SoftSwitch is a Class 4 and Class 5 softswitch with a capacity of up to 1,000 CAPS (Call Attempts Per Second). It handles:

• Intelligent call routing: rules based on prefix, time-of-day, cost, or quality (LCR/ASR)
• Authentication and authorization: granular control per customer, IP address, or SIP credential
• Carrier interconnection: configuration for interconnection with licensed telecom operators
• Wholesale and retail: the same softswitch supports both wholesale resale and direct delivery to end customers
• Real-time CDR: detailed records for every call, feeding billing and audit systems

With 1,000 CAPS, the SipPulse SoftSwitch can handle signaling volumes compatible with large regional operators, with no bottleneck in call establishment.

SipPulse SBC: Border Security and Control

The SipPulse SBC supports up to 4,000 concurrent calls and is available in three variants, each optimized for a specific scenario:

• SBC UNI (User-to-Network Interface): designed for customer access. It interfaces between the enterprise PBX and the ISP's network, handling NAT traversal, signaling normalization, and call admission control (CAC).
• SBC NNI (Network-to-Network Interface): designed for carrier interconnection. It manages SIP traffic between the ISP and other carriers or the PSTN.
• SBC NNI-CC (Contact Center): optimized for contact center scenarios with high concurrent call volumes and specific routing requirements.

Additionally, the SipPulse SBC offers:

• STIR/SHAKEN: caller identity authentication, essential for regulatory compliance and fraud prevention
• TLS and SRTP: signaling and media encryption to protect voice traffic
• Native WebRTC: allows customers to connect browser-based softphones directly to the SBC
• Zoom and Microsoft Teams integration: interoperability with unified communication platforms

SipPulse BSS/BCORE: Billing and Subscription Management

The SipPulse BSS (Business Support System), also known as BCORE, is the billing and subscription management system that auto-syncs with the SoftSwitch. It solves one of the biggest challenges for ISPs entering the voice market: how to bill correctly.

• Automatic CDR import: call records from the SoftSwitch are imported and rated automatically
• Flexible billing plans: per-minute, flat rate, bundled, or hybrid
• Subscription management: control of the customer lifecycle, from provisioning to cancellation
• Revenue management: dashboards for invoicing, delinquency tracking, and per-customer profitability

How the Three Products Work Together

The differentiator of the SipPulse platform for ISPs is the native integration between the three components:

1. The SipPulse SBC receives the call from the enterprise customer and applies security and normalization policies
2. The SipPulse SoftSwitch routes the call to the correct destination (another carrier, PSTN, or internal)
3. The SipPulse BSS records usage and applies billing according to the contracted plan

This integration eliminates the need for custom scripts between systems from different vendors. Provisioning a new customer can be done end-to-end: when a customer is created in the BSS, channels are automatically configured in the SoftSwitch and security policies are applied on the SBC.

Structuring the SIP Trunking Product

With the SipPulse platform, the ISP can structure SIP trunking plans clearly and with full automation:

• Concurrent channels: number of simultaneous calls contracted (the SBC supports up to 4,000 per instance)
• DIDs (Direct Inward Dialing): inbound numbers configured directly in the SoftSwitch
• Included minutes: minute bundles for local, long-distance, and international calls, with automatic rating by the BSS
• Additional services: call recording, IVR, call queuing

Example Plans

The SipPulse BSS allows you to create these plans with custom rating rules and automatically control minute overages.

Billing Models

The main billing approaches, all natively supported by SipPulse BSS, are:

• Per-minute: charges based on actual usage, with differentiated rates by destination (local, long-distance, international, mobile). The BSS imports CDRs from the SoftSwitch and applies rating automatically.
• Flat rate: fixed monthly fee with unlimited minutes to certain destinations. The BSS monitors usage and can apply fair-use policies.
• Bundled: combination of fixed channels with included minute packages and per-minute overage. The most common model in the market.

Numbering and E.164

To offer SIP trunking with geographic numbers (DIDs), the ISP needs to:

• Obtain numbering ranges from the local regulator (or via agreement with an operator that holds the numbering)
• Configure numbers in E.164 format (with country code and area code) directly in the SipPulse SoftSwitch
• Implement number portability via the local portability clearinghouse
• Assign DID ranges linked to the customer's business identity

The SipPulse SoftSwitch has native support for portability lookups and E.164-based routing.

Regulatory Requirements

Depending on the jurisdiction, offering voice services with proprietary numbering may require:

• Telecommunications license: authorization from the local regulator to provide voice services. SipPulse works with multiple licensed operators and can guide ISPs through this process.
• Interconnection: interconnection agreements with destination operators, managed through the SipPulse SBC NNI
• Number portability: integration with the national number portability system, natively supported by the SoftSwitch
• Quality obligations: compliance with quality indicators defined by the regulator
• STIR/SHAKEN: the SipPulse SBC already supports this caller identity framework, which is increasingly required by regulators worldwide

Alternatively, the ISP can act as a reseller for a licensed voice operator, simplifying regulatory requirements. The SipPulse platform works in both models.

Quality of Service (QoS)

The competitive advantage of an ISP offering SIP trunking over its own internet links is the ability to guarantee quality:

• Dedicated bandwidth: reserve bandwidth for voice traffic, separated from data traffic
• DSCP marking: mark voice packets with EF (Expedited Forwarding, DSCP 46) for prioritization in routers
• Jitter and latency monitoring: keep jitter below 30ms and latency below 150ms
• Packet loss: keep below 1% for acceptable voice quality

The SipPulse SBC monitors call quality in real time and can generate automatic alerts when QoS indicators degrade.

Automated Provisioning with SipPulse

With the integrated SipPulse platform, provisioning new SIP trunking customers is automated end-to-end:

1. The ISP creates the customer and plan in SipPulse BSS
2. The BSS automatically provisions channels and routing rules in SipPulse SoftSwitch
3. DIDs are assigned and configured in the SoftSwitch
4. Security and CAC policies are applied on the SipPulse SBC
5. SIP credentials are generated and made available to the customer
6. Monitoring and billing are activated automatically

This workflow eliminates manual provisioning, reduces errors, and allows the operation to scale without proportionally increasing the technical team.

Monitoring and SLA

A professional SIP trunking service should include:

• Real-time monitoring of active calls and answer-seizure ratio (ASR)
• Automatic alerts for quality degradation (MOS below 3.5)
• Customer dashboard to track usage and quality (provided by SipPulse BSS)
• Contractual SLA with availability targets (99.9% or higher)
• Defined incident handling process

Conclusion

SIP trunking is a natural extension of the portfolio for an ISP that already offers connectivity. The SipPulse platform, with the SoftSwitch for routing, the SBC for border security, and the BSS for billing and subscription management, provides everything an ISP needs to launch and scale a professional voice operation. The native integration between the three components eliminates the complexity of integrating systems from different vendors and lets the ISP focus on what matters: winning and retaining business customers.

References

• https://datatracker.ietf.org/doc/html/rfc3261
• https://datatracker.ietf.org/doc/html/rfc4566
• https://datatracker.ietf.org/doc/html/rfc3680
• https://www.itu.int/rec/T-REC-E.164