SIP Trunk or Primary Multiplex—Which Is Right for You?
A primary multiplex connection with 30 voice channels was long the standard for businesses with high call volumes. Today, when upgrading systems, the question that usually arises is: SIP Trunk or Primary Multiplex? It's not just about the type of connection. This decision affects ongoing costs, flexibility as your business grows, integration with Microsoft Teams, and how reliably your employees can be reached at every location.
For many Swiss SMEs, the SIP trunk is the more cost-effective and future-proof choice. However, there are situations in which an existing primary multiplex connection remains a sensible option for the time being. The key factors to consider are your phone system, your Internet connection, your availability requirements, and a well-planned migration path.
What is the difference between a SIP trunk and primary multiplex?
The Primary Rate Interface (PRI), also known as E1, is a circuit-switched telephone connection from the ISDN world. One connection typically provides 30 voice channels that can be used simultaneously. The capacity is fixed: If a company needs more channels, it requires additional connections or upgrades. The voice connection runs over a separate, traditional telecommunications infrastructure.
A SIP trunk, on the other hand, transmits calls over IP. It connects an IP phone system, a cloud PBX, or a session border controller to the public telephone network. The number of concurrent calls is agreed upon as channel capacity and is generally much easier to adjust. Instead of a fixed bundle, companies can tailor their telephony to actual usage, seasonal peaks, or new locations.
Important: A SIP trunk is not the same as a PBX. It is the connection to the public network. Features such as IVR, call queues, group calls, presence indicators, CTI, or integration with Microsoft Teams are provided by the PBX. Therefore, especially when modernizing a system, the entire communications architecture should always be taken into account.
SIP Trunk vs. Primary Multiplex: Costs and Scalability
With primary multiplexing, costs are often incurred for capacity that isn’t needed all the time. If you order 30 channels, you pay for the technical setup regardless of whether there are five or thirty calls taking place simultaneously at noon. For companies with a consistently high and very steady call volume, this might make sense. For growing SMEs, project-based organizations, or businesses with a hybrid work model, however, this model is usually not very flexible.
With a SIP trunk, the number of concurrent calls can be scaled more effectively to meet actual demand. A company with 18 employees does not automatically need 18 voice channels. The key factor is how many people realistically make external calls at the same time. Analyzing peak times helps prevent both unnecessary monthly costs and busy lines in customer service.
Scaling also involves more than just channels. Whether it’s a new location, additional work-from-home stations, or a temporary campaign, an IP-based solution can be expanded much more quickly. Phone numbers, extensions, and rules can be managed centrally. This reduces the workload on the IT department and makes costs predictable, provided that the rate plan, call volume, and desired flat rates are clearly defined.
Internet connectivity is becoming a decisive factor
A primary multiplex connection features a voice connection that is separate from data traffic. That was one of its practical advantages: even with heavy Internet traffic, voice transmission remained unaffected. With a SIP trunk, voice and data often share the same IP infrastructure. This works very reliably if the network and security are properly planned—but not automatically.
Good voice quality requires sufficient bandwidth in both directions, low latency, minimal jitter, and prioritized transmission of voice packets. Quality of Service, well-segmented networks, and a properly configured firewall are therefore essential components of the solution. When there are multiple locations, site-to-site connectivity is also important. A fast connection at headquarters is of little use if a branch office is making calls over an unstable connection.
A Session Border Controller protects and manages SIP communication at the network boundary. Among other things, it prevents unauthorized access, ensures compatibility between the phone system and the service provider, and can enforce rules for phone numbers and call routes. Depending on the architecture, an SBC can be operated locally or provided as a managed component. Not every small business needs a complex setup, but no business should do without professional security measures.
Reliability: It's Not Just About the Connection
The statement „Primary multiplex is more secure than SIP“ does not tell the whole story. A single PRI connection can fail just as easily as a single Internet line. What matters is whether there is redundancy and what happens in the event of a failure.
With SIP, emergency plans can often be implemented very flexibly. If the Main line from, incoming calls can be redirected to cell phones, another location, or a second telephony platform. With two independent Internet connections and an appropriate failover configuration, call availability remains much easier to manage even in the event of line problems. This represents a tangible business advantage for reception, support, or on-call services.
Responsibilities must be clearly defined. Who monitors the network, firewall, SBC, and phone system? Who adjusts call forwarding settings when a location closes? And how quickly can a contact person be reached in the event of a malfunction? Technology and service go hand in hand. A low connection fee is of no help if, in the event of an outage, no one has an overview of the entire situation.
Microsoft Teams and Modern Telephony
Anyone who uses Microsoft Teams in their day-to-day work should align their connectivity choices with their collaboration strategy. A primary multiplex cannot directly support Teams. This requires additional components, gateways, or a PBX system that connects the traditional ISDN environment with IP telephony. While this is technically possible, it increases complexity and maintenance efforts.
A SIP trunk, on the other hand, fits right into modern communication environments. Employees can use their business phone number on a desk phone, softphone, or mobile device or teams be reachable. Presence information, internal calls, and external calls can be effectively integrated. This requires an architecture that clearly maps phone numbers, emergency call requests, access permissions, and failure scenarios.
For companies that want to use Teams as a full-fledged telephony channel, a cloud PBX with native Teams integration is a good option. Winet, for example, connects the Ayrix PBX directly to Microsoft Teams without the need for separate Teams add-ons or additional SBC components that require a license. Whether this model is right for your company depends on your existing system infrastructure and the features you need.
Migration from the Primary Multiplex: Step-by-Step Rather Than Risky
A transition doesn't have to mean changing all phone numbers and devices at once on a Friday evening. A phased approach makes sense, especially for businesses with reception areas, call centers, alarm systems, elevator phones, fax machines, or industry-specific applications.
The first step is to take stock: What phone numbers and extensions are in use? How many calls are active at the same time? Which devices require analog interfaces? Which applications use telephony via CTI, SIP, or ISDN? Special cases such as door intercoms, payment terminals, and emergency call devices must also be documented. These often determine whether adapters, gateways, or separate connections are necessary.
Next, the new capacity is planned, the Internet quality is checked, and a test environment is set up. Phone numbers can In many cases, these numbers can be ported so that customers can continue to dial the numbers they are familiar with. For the actual switchover day, a schedule is needed that outlines responsibilities, includes tests for incoming and outgoing calls, and specifies clear fallback rules. After the system goes live, call quality, load, and routing should be monitored for several weeks.
When Primary Multiplexing May Still Be Useful
A switch to SIP is not always immediately necessary. If you have a large, specialized telephone system with many ISDN cards, a transition phase may be more cost-effective than a hasty, complete overhaul. Even at locations without a sufficiently reliable IP connection, the network infrastructure must first be improved or redundant access established.
However, anyone who is already replacing a PBX, integrating Microsoft Teams, professionally integrating remote work, or looking to centrally manage multiple locations should carefully evaluate the investment in additional PRI technology. New traditional hardware ties up capital and prolongs dependence on an architecture that offers little flexibility for modern ways of working.
The right choice isn't based on a one-size-fits-all recommendation, but rather on your call types, existing infrastructure, and desired operating model. If you honestly assess these factors before the migration, switching from primary multiplex to SIP won't be a risky technology change—it will be a controlled improvement to your day-to-day availability.