For decades, businesses had little choice but to rely on the legacy public switched telephone network (PSTN) when connecting their private branch exchange (PBX) phone systems. This meant paying for a fixed number of physical phone circuits from the telco, with almost no flexibility to scale. But in the internet age, SIP trunking offers a modern replacement that can significantly cut costs. Here’s a closer look at how SIP trunking leaves old-fashioned PSTN in the past.
Playing Telephone Tag with PSTN Trunks
The traditional PBX phone system design relied on point-to-point connections with the outside world. To make this happen, your business would order a number of phone trunks from the phone company to plug into your PBX. These connections came in the form of analog phone circuits or digital ISDN/T1 lines.
Once in place, these PSTN trunks enabled users on your PBX system to make and receive calls. But here was the major catch: you couldn’t quickly add or remove trunk capacity. It typically took weeks of lead time with the telco to provision more circuits. And you were generally locked into long contracts.
This lack of agility led most organizations to overbuy trunk capacity. After all, no one wanted callers to get a busy signal during peak demand. But all those extra unused trunks came at a steep price.
Paying for Phantom Phone Traffic
To prevent busy signals during busy hour, most businesses wound up over-provisioning PSTN trunks by 50% or more. This created significant expense for capacity that sat idle nearly 95% of the time.
Very seldom did anyone reevaluate telephony needs outside of initial installation or a major upgrade. And the phone company wasn’t about to volunteer cost-saving advice. Legacy telcos preferred to keep customers trapped on autorenewing PSTN trunk contracts negotiated years or decades earlier.
Escaping this cycle required proactively monitoring PBX call records to see actual utilization. But relatively few organizations went through the hassle when the shortcomings of PSTN trunks seemed unfixable.
Shedding the Fixed Line Mentality
Part of what made PSTN trunk management so rigid was the dependence on fixed physical infrastructure. Analog phone circuits and T1/ISDN lines didn’t allow for flexible capacity adds or changes. You couldn’t “burst” to temporarily handle spikes in call volumes.
Meanwhile, mobile and internet powered innovation increasingly left PSTN in the dust. End users came to expect flexibility that traditional telephony couldn’t deliver. As PBX systems edged closer to extinction, many wondered if there might be a bridge to the future.
SIP Trunking Bridges the Way to Nimble Networks
The rise of voice over IP (VoIP) and IP-based PBXs finally offered an escape route. These refreshed PBX platforms allowed connectivity over general purpose IP data networks instead of depending on rigid PSTN infrastructure.
The signaling protocol enabling this new flexible connectivity is Session Initiation Protocol (SIP). On the PBX side, SIP trunks replace legacy analog or digital tie lines. On the service provider side, connections shift from PSTN to robust VoIP networks.
This sets the stage to treat voice just like any other application. Call capacity can scale up or down dynamically based on policies aligned to actual needs rather than arbitrary ratios. The overprovisioning and idle capacity waste of old finally has a cure.
Right Sizing SIP Trunk Capacity
Moving from PSTN to SIP offers many advantages. But to maximize savings, its crucial to accurately calculate required SIP trunk capacity at the outset. Then set baseline needs at the minimum functional level, relying on the ability to “burst” upward as needed.
The starting point is learning current usage based on records and reporting from your legacy PBX system. While painful to gather, this call volume data holds the key to make smarter buying decisions. It can show busy hour traffic patterns that lead the way to precision SIP trunk provisioning.
Another key tool for capacity planning comes from telephony engineering models and calculators. Online tools like Erlang B/C help convert call traffic patterns into capacity profile targets, factoring in the desired success rate. Though complex under the hood, their output is SIP trunk number firm targets to better match actual needs.
Staying Flexible with SIP
Unlike rigid and expensive PSTN links, SIP trunking offers flexibility akin to cloud computing services. Through rapid provisioning tools, capacity can increase or decrease almost instantly in response to changing call patterns or organizational needs.
Pre-buying fixed capacity takes a backseat to only paying for what you use. Spikes get handled by automatically “bursting” up temporarily as needed. But bursting only drives incremental cost versus excess fixed capacity contracts. This makes the economics far more forgiving than old telephony models.
With major mobile and cloud disruption, users expect flexible services. SIP trunking brings public telephony along for the ride, unshackling businesses from rigid infrastructure. By connecting PBX platforms to robust VoIP networks, SIP offers scalable, agile capacity at lower costs. That makes it high time to hang up PSTN for good. The savings await any business ready to cut the cord.