Glossary · SIP
What is SIP trunking?
SIP trunking is a method for delivering voice and messaging services over the internet using the Session Initiation Protocol (SIP). It replaces traditional analog phone lines with virtual “trunks” that connect a customer’s on-premises PBX or Session Border Controller (SBC) to a VoIP provider, which then connects to the Public Switched Telephone Network (PSTN). SIP trunking is common in enterprise deployments where the customer wants to keep an existing PBX but gain cloud flexibility and lower costs.
How SIP trunking works
- Customer operates an IP-PBX or SBC on-premises or in a private cloud
- SIP trunks run between that PBX/SBC and the SIP trunk provider
- The SIP provider handles PSTN origination and termination
- Calls traverse the trunks as SIP signaling + RTP media streams
- Providers allocate DIDs (Direct Inward Dialing numbers) to the customer
The “trunk” terminology comes from analog telephony where physical trunks carried multiple lines between switches. SIP trunks carry multiple concurrent calls over IP.
SIP trunking vs. hosted VoIP / UCaaS
| Dimension | SIP Trunking | Hosted VoIP / UCaaS |
|---|---|---|
| PBX location | Customer-operated (on-prem or private cloud) | Provider-operated |
| Setup complexity | Moderate to high (SBC config, routing) | Low (sign up, download app) |
| Control | Full control over PBX features and routing | Provider controls platform, customer controls config |
| Infrastructure cost | Customer pays for PBX/SBC | No infrastructure cost |
| Feature breadth | Depends on customer’s PBX | Full UCaaS stack from provider |
| Best for | Enterprises with existing PBX investments | Small to mid-market; simplicity buyers |
DialPhone delivers hosted VoIP / UCaaS natively. For customers with existing SBC investments or complex routing requirements, DialPhone also supports SIP trunking to feed a customer-operated PBX or to enable Microsoft Teams Direct Routing.
Why use SIP trunking
- Preserve existing PBX investment — don’t replace a working on-premises system
- Complex routing requirements — survivable branch appliances, regional failover, specialized recording
- Regulatory control — keep media paths within a specific jurisdiction
- Hybrid deployment — gradually migrate from on-prem PBX to cloud
- Microsoft Teams Direct Routing — use a SIP trunk through a customer-operated SBC
- Custom integration — telephony integrated with in-house applications
SIP trunking pricing models
- Per concurrent call (channel) — pay for simultaneous call capacity
- Per-user — flat per-user pricing regardless of call volume
- Per-minute — metered pricing based on usage
- Unlimited per channel — flat per-channel with unlimited usage
DialPhone SIP trunking pricing is available on request and varies by region, committed volume, and included DID count. The hosted VoIP model (per user) covers most use cases and is significantly simpler to operate.
Technical components
- SIP — the signaling protocol that sets up, modifies, and terminates sessions
- RTP / SRTP — the media protocol that carries the actual audio (Secure RTP for encryption)
- SBC (Session Border Controller) — security and interoperability gateway at the edge of the customer’s network
- Codec — compression algorithm (G.711, G.722, Opus) that encodes voice into packets
- DID — Direct Inward Dialing number, the phone number assigned to each trunk or user
- DTMF — touch-tone digits sent via SIP INFO or RFC 2833
Example
A 2,000-employee financial services firm has a Cisco Unified Communications Manager (CUCM) PBX deployed across three data centers. They use SIP trunking from DialPhone to:
- Replace their previous TDM (analog) trunks with SIP trunks, cutting $40,000 per year in PSTN line costs
- Maintain their existing CUCM dial plan and call routing
- Enable Microsoft Teams Direct Routing for a subset of users moving to Teams Phone
- Keep sensitive financial calls routed through the on-premises CUCM for FINRA compliance
See SIP trunking options
Most customers don’t need SIP trunking — hosted UCaaS is simpler and cheaper. SIP trunking fits specific enterprise scenarios. Talk to sales → for SIP trunking pricing and deployment details.