What Is SIP Trunking? SIP Trunks Explained
A phone system that can only call itself isn't much use. The moment you want to dial a real phone number, you need a bridge to the outside world — and that bridge is also where a mis-configured system quietly haemorrhages money.
A SIP trunk is a virtual connection between your IP phone system and a telephony provider (ITSP) that gives you access to the public telephone network — inbound numbers and outbound calling — over IP instead of physical phone lines.
What replaced the copper
Traditional phone lines (PRI, analogue) came in fixed bundles of channels and had to be physically installed. A SIP trunk delivers the same thing — the ability to make and receive external calls — as a service over your data connection. You buy capacity (concurrent calls) and phone numbers (DIDs), and scale them up or down without new wiring.
Most organisations now mix trunk types: SIP trunks for most traffic, with PRI or FXO/FXS gateways where legacy lines or analogue devices still exist.
Where the money leaks
A SIP trunk is a billing relationship: calls that go out over it cost money, often per-minute to international and premium-rate destinations. If an attacker breaks into your phone system, the trunk is how they cash out — by placing thousands of calls to expensive numbers before you notice. This is toll fraud, and it can run to tens of thousands of dollars overnight.
The defence is to treat the trunk as privileged: restrict which destinations are allowed, cap concurrent and daily call volume, and secure the trunk itself with TLS and SRTP where your provider supports it.
Secure trunking vs plain trunking
Not every provider supports encryption, so good systems let you choose per-provider: encrypted transport (TLS) and encrypted media (SRTP) where available, plain where it isn't. The point is that it's a deliberate choice you can see and control — not an invisible default sending your calls in clear text.
Frequently asked questions
How many SIP trunk channels do I need?
Size them to your busy-hour concurrent calls, not your total users — only a fraction of extensions are on external calls at once. A sizing exercise against your call patterns gives the real number.
Can SIP trunks be encrypted?
Yes, where the provider supports TLS for signalling and SRTP for media. It should be selectable per provider, since not every carrier offers it.