A SIP Trunk is the connection that links your phone system to the public telephone network over the internet, instead of physical lines. It is the modern way to make and receive calls.
This guide explains what a SIP Trunk is and how it works, without focusing on any one market. If you already know that and want local numbering, porting and pricing in Costa Rica, go to SIP Trunk in Costa Rica: a business guide.
How it works
Instead of contracting physical lines or trunks (E1, analog), the SIP Trunk carries calls as data over the internet using the SIP protocol. Your PBX —physical, virtual or cloud— connects to the provider, and phone traffic flows in and out from there.
Advantages over traditional lines
- Savings: removes physical lines and lowers national and international call costs.
- Scalability: add or remove channels on demand, with no physical work.
- Flexible numbers: local numbers from several areas and toll-free, no matter where your office is.
- Continuity: with managed routes and failover, calls keep flowing even if a link fails.
Who a SIP Trunk is for
It is ideal if you already have a PBX (for example FreePBX/Asterisk or an IP-PBX) and want to modernize connectivity without changing your whole system. If instead you want a complete no-hardware solution, consider UCaaS.
See the details on our SIP Trunk page.
How many channels you need
A SIP Trunk is not bought "per line" but per concurrent channel: how many calls you can have at the same time. The classic mistake is asking for one channel per employee, which means paying too much.
Rule of thumb for a normal office: roughly 1 channel per 3 to 5 users. In a contact center, where almost everyone is talking at once, it approaches 1 per agent plus a margin. If you are unsure, look at how many simultaneous calls you had in last month's peak hour: that number plus 20 % is a good starting point.
The advantage over physical lines is that getting it wrong is cheap: channels are added or removed in minutes, with no cabling and no site visit.
What to check on the network
Most SIP Trunk problems are network problems, not telephony problems:
- Bandwidth: around 100 kbps per call each way. Ten simultaneous calls is about 1 Mbps. Not much, but it has to be there.
- Latency, jitter and loss: what really decides whether audio sounds good. A fast but unstable link sounds worse than a slow, steady one.
- Voice prioritisation (QoS): so a large download does not chop a call.
- NAT and firewall: the classic cause of "the call connects but there is no audio". Fixed with correct router configuration, not by lowering security.
Security: toll fraud is real
A poorly protected trunk is a surprise invoice. Attacks look for weak credentials to place calls to expensive destinations, usually overnight and at weekends. The basic defences:
- Strong credentials and registration only from authorised IPs.
- Encrypted signalling with TLS and audio with SRTP.
- Spend and channel caps, so any abuse hits a ceiling quickly.
- Blocking international destinations your company never calls.
- Unusual-usage alerts, which is what actually catches the problem in time.
What to ask before signing
- Are the numbers in your company's name and portable?
- Is there an alternate route if the primary fails?
- How is it billed: per channel, per minute, or a bundle?
- Are there configurable spend limits?
- Who do you talk to when there is an audio problem mid-morning?
Frequently asked questions
Does it work with my current PBX?
Yes. The SIP Trunk connects to physical, virtual or cloud PBXs that support SIP.
How many simultaneous calls does it support?
As many as you need: defined by channels and scaled to your traffic.
Can I have numbers from other countries?
Subject to availability, yes; local and toll-free numbers from different areas can be enabled.
Does it work with FreePBX, Asterisk or 3CX?
Yes. A SIP Trunk works with practically any SIP-capable phone system, physical, virtual or cloud.
How much bandwidth does it use?
Around 100 kbps per call each way. What is critical is link stability, not raw speed.
What happens if my internet goes down?
With backup routes, inbound calls divert to another destination (mobile or another site) so the operation keeps running.
How do I prevent toll fraud?
Strong credentials, IP-based registration, encryption, spend caps, blocking destinations you never call, and alerts on unusual usage.