Getting a SIP Trunk: How it works and what you need

A SIP Trunk is an internet-based voice connection between your PBX (Private Branch Exchange) and the public telephone network. Your PBX sends call signalling to a provider using the SIP protocol, which then routes the calls to any landline or mobile phone.

It replaces ISDN or PRI lines (traditional physical telephone lines), and the number of simultaneous calls depends on the channels you subscribe to. If your call traffic varies significantly, an elastic SIP Trunk allows you to increase capacity on demand.

Quick glossary

How a SIP Trunk works

SIP Trunk technology works over a network, converting voice into data packets so it can be transmitted over the internet. With an internet connection, you can make and receive calls through a SIP channel.

Calls originate from the company's virtual PBX and travel over the internet through the SIP Trunking channel, which routes them to a landline or mobile phone. It can establish a connection channel for small PBX systems or support an unlimited number of connections for large companies and call centres.

Two types of traffic are involved in every call:

• Signalling (SIP): The messages that establish and terminate the call (who is calling, who they are calling and how the voice will be compressed).
• Voice (RTP): The data packets that carry the conversation in both directions for the duration of the call.

When you make a call, the following happens:

1. The voice is converted into data.
2. The call is sent over the internet.
3. The SIP provider connects the call.
4. The voice reaches the recipient.

Incoming calls work in reverse: the phone number (DDI) assigned to your SIP Trunk receives the call, the provider sends it to your PBX, and your PBX routes it to the agent, voice menu or extension you have configured.

SIP Trunk vs. Traditional Phone Lines

Before SIP, businesses subscribed to physical lines (ISDN or PRI), which had a limit of 30 voice channels on a single link. They were reliable, but required cabling, dedicated equipment and subscriptions to fixed bundles of lines.

What do you need for a SIP Trunk to work?

For a business to use this service, it needs the following:

• A SIP-compatible telephone system (PBX).
• A high-quality internet connection. Stability is important to ensure voice calls have no delays or interruptions.
• A VoIP telephony provider, such as Megacall.
• An IP phone or softphone connected to the PBX.

What to consider when using a SIP Trunk

Codecs and Bandwidth

A codec is the system that compresses voice so it can be transmitted over the internet. It determines audio quality, while bandwidth refers to the capacity of your internet connection used by each call.

Codecs such as G.711 offer good sound quality but require more bandwidth than alternatives such as G.729. Therefore, you should choose an option compatible with your provider and make sure your connection can support the number of simultaneous calls you need without interruptions or delays.

As a reference, 20 simultaneous calls using G.711 require almost 2 Mbps of upload and download bandwidth for voice alone.

Security

A SIP Trunk is exposed to the internet and, like any internet-connected service, can be targeted by fraud attempts. These may involve calls to premium-rate numbers or expensive international destinations, resulting in high bills. To prevent this, you should strengthen several security measures:

• Strong authentication. Only allow access from your company's IP address and use long, unique passwords.
• Firewall. This acts as a filter to protect your network. Configure it to accept connections only from your provider on the ports used for voice traffic.
• Encryption. If your provider supports it, enable TLS encryption for call signalling and SRTP for voice traffic to prevent conversations from being intercepted or manipulated.
• Spending limits and blocked destinations. Disable countries you don't call and set up usage alerts.
• Keep your PBX updated and check your call history regularly.

Numbering and number portability

Check whether you can keep your existing numbers through number portability or get new geographic numbers, as well as national or international numbers. You should also verify that you can make and receive calls to and from the destinations your business

Number of simultaneous calls

Check how many calls your SIP Trunk can handle at the same time. This limit depends on the channels you subscribe to and should match your business's call volume, especially during peak hours.

Advantages of using a SIP Trunk

• Cost savings: SIP call rates are significantly lower than those of traditional telephony. In addition, fixed monthly fees are reduced or eliminated, and you only pay for the minutes you use. Costs associated with incoming calls, long-distance calls, IT and maintenance can also be significantly reduced or eliminated.
• Flexibility: SIP calling allows one or more phone numbers to be connected to multiple telephone lines, giving you the option to add lines or channels according to your business's call volume. This is an advantage over traditional telephony, which is more rigid and expensive because it requires businesses to subscribe to a set number of lines in fixed bundles.
• Remote work: SIP Trunking and the virtual PBX are connected over the internet, allowing employees to work remotely and make calls from anywhere in the world. Distance is no longer an issue, and call costs remain the same.
• Integration: A SIP Trunk can be integrated with a PBX or virtual PBX. This also allows you to connect a CRM, ERP, your company's own applications and call centre software.
• Security and reliability: This is a highly reliable technology. Since it doesn't rely on traditional telephone cabling, it avoids certain electrical faults and circuit failures. In addition, calls can be redirected to alternative data lines to ensure incoming calls continue if there is a problem with your current provider.

What is an elastic SIP Trunk?

An elastic SIP Trunk is a flexible, scalable SIP Trunk. The number of simultaneous calls is not fixed in advance but increases or decreases according to your needs at any given time, and you only pay for what you use.

The difference compared with a conventional SIP Trunk is easy to understand with an example: an online store that normally handles 10 simultaneous calls but receives 40 during Black Friday. With a conventional SIP Trunk, it would have to subscribe to 40 channels all year, paying for capacity it rarely uses, or stick to 10 and risk customers hearing a busy signal once the eleventh call comes in.

With an elastic SIP Trunk, capacity increases during the campaign and returns to its normal level afterwards, without changing plans. The same applies to call centres, whose capacity can automatically adapt to changes in activity during peak seasons.

When is it useful?

• Sales campaigns with peaks concentrated over a few days.
• Seasonal businesses, such as tourism companies, or businesses experiencing peak demand during sales periods and Black Friday.
• Call centres that increase or reduce the number of agents throughout the year.
• Growing businesses that don't want to renegotiate their capacity every few months.

What is Megacall's SIP Trunk model?

Megacall offersSIP Trunking with no setup fee or fixed monthly cost, pay-per-minute calling and up to 50 free simultaneous channels. It works with most SIP platforms, and the main providers and systems we work with at Megacall include:

• Cisco SIP Trunk
• Avaya SIP Trunk
• Asterisk SIP Trunk
• Elastic SIP Trunk
• 3CX SIP Trunk
• FreePBX SIP Trunk

Elastic SIP Trunking with external agents at Megacall

One of the most common use cases is connecting a virtual PBX to an AI voice agent to handle incoming calls automatically. In this case, the process works as follows:

1. The customer initiates a call from their personal phone by dialling your public number.
2. Megacall receives the call on its servers because you own that virtual number.
3. The SIP Trunk immediately acts as an invisible bridge, forwarding the audio from the incoming call to the IP address or URI of the AI platform you have configured.
4. The AI platform (Retell, Vapi or ElevenLabs) processes the audio, activates the virtual agent and responds to the customer in real time through the same channel.

What does Megacall's SIP Trunk service include?

• Free setup and immediate online access.
• Up to 50 simultaneous voice channels.
• Control over the displayed number (Caller ID or CLI).
• G.711A, G.711U and G.729 codecs.
• National and international phone numbers.

How to get a SIP Trunk with Megacall

Getting a SIP Trunk is straightforward. You just need to follow these steps:

1. Register your account on the Megacall website. A SIP Trunk is created automatically by default.
2. If needed, you can also create your virtual PBX to manage your calls.
3. Finally, you can purchase virtual numbers by submitting all the documentation required for activation.
4. Once your numbers are available, you can assign them to your SIP Trunk or virtual PBX.

FAQs - SIP Trunk

What is a SIP Trunk in simple terms?

It is an internet connection that links your virtual PBX to the public telephone network, allowing you to make and receive calls while replacing traditional physical phone lines.

What is the difference between a SIP Trunk and a PBX?

A virtual PBX manages extensions, IVR menus, call queues and other features, such as analytics, call recording and speech recognition.

A SIP Trunk is the connection that allows the PBX to make and receive calls over the public telephone network.

Is it safe to use a SIP Trunk for business calls?

Yes, provided it is configured correctly and appropriate security measures are in place. We recommend using secure authentication, restricting authorised connections, protecting signalling and audio with TLS and SRTP when available, and monitoring traffic to detect potential fraudulent activity.

How do I know if my business needs a SIP Trunk?

It may be a good option if your business uses an IP PBX and needs to handle a high volume of calls or integrate telephony with other systems.