SIP trunk: definition, benefits and implementation in business
What is a SIP trunk, how many channels do you need, how do you keep your numbers and secure your calls? A practical guide for Swiss businesses.
The SIP trunk has become the standard way to connect a business to the phone network. Since Swisscom phased out ISDN technology, fixed-line telephony in Switzerland runs entirely over IP. But what exactly does the term cover, and how do you size it correctly?
What is a SIP trunk?
A SIP trunk is a virtual phone connection between your phone system and an operator, running over your internet connection or a dedicated data line. SIP (Session Initiation Protocol) is the protocol that sets up and ends calls.
In practice, the SIP trunk replaces old physical lines: instead of running one cable per line, the operator provides a number of channels, i.e. simultaneous calls, and a set of numbers.
A SIP trunk connects to:
- an IP phone system on premises or in the cloud, such as 3CX;
- Microsoft Teams, via a session border controller or a service such as our Open Connect for Teams Phone;
- an older phone system, using a gateway.
The benefits for businesses
Lower, clearer costs
No more physical lines to rent: you pay for channels and calls, per minute or at a flat rate. Calls between your sites go over your network, and international calls are generally cheaper.
Flexibility
Adding channels for a busy period or a new site takes a few days, with no building work. You can also assign numbers from other countries to establish a local presence with your customers.
Service continuity
If a site goes down, calls can be automatically redirected to another site or to mobiles. Your customers never hear an engaged tone.
Keep your numbers
In Switzerland, number portability lets you keep your numbers when you change operator. The switchover is planned and carried out without interruption.
How many channels do you need?
The number of channels corresponds to the maximum number of simultaneous external calls, not the number of users. To estimate it:
- Check the number of simultaneous calls at peak times on your bills or your current phone system.
- Add a margin for growth and seasonal peaks.
- Plan dedicated channels if you have a call centre or a very busy switchboard.
| Profile | Rough guide |
|---|---|
| Standard office | 1 channel per 3 to 5 users |
| Sales or support department | 1 channel per 1 to 2 users |
| Call centre | 1 channel per agent, plus a margin for queues |
These ratios are indicative: analysing your actual traffic remains the best reference.
Securing your SIP trunk
A poorly protected SIP trunk can be hijacked to make fraudulent calls to premium-rate numbers, often at night or at weekends. A few simple rules:
- place a session border controller (SBC) between your network and the operator, for example AudioCodes Virtual SBC;
- encrypt signalling and voice (TLS and SRTP) when the operator supports it;
- restrict permitted destinations (countries, premium-rate numbers);
- monitor usage and set up overrun alerts.
Best practice: block destinations you do not need by default and open them on a case-by-case basis.
The most common use cases
- Multi-site businesses: a centralised connection for all offices.
- Call centres: channels matched to activity, with no commitment to physical lines.
- Phone system migration: temporary numbers to test the new solution before porting the permanent numbers.
- Remote working: staff call using the company number, wherever they are.
In summary
The SIP trunk is the foundation of modern business telephony: cheaper, more flexible and compatible with all current solutions. To get the most out of it, size it on your actual traffic and secure it from day one.
Our operator services provide SIP trunks, numbers in more than 20 countries and porting of your current numbers. Request a quote tailored to your sites.
