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What "99.95% Uptime" Actually Means: How to Read a Phone System SLA
Every provider’s website has an uptime number. Almost nobody reading it knows what it commits the provider to — including, sometimes, the provider.
Your phone system is the one service where downtime is immediately, painfully visible: callers get dead air, and every minute is a customer who thinks you’ve closed. So before you weight an uptime percentage in a buying decision, it’s worth knowing how to read one. Here’s the working guide.
First, the arithmetic
Section titled “First, the arithmetic”An uptime percentage is only meaningful once you convert it to time. Over a 30-day month:
| Commitment | Allowed downtime / month | Allowed downtime / year |
|---|---|---|
| 99% | ~7.3 hours | ~3.7 days |
| 99.9% | ~43 minutes | ~8.8 hours |
| 99.95% | ~22 minutes | ~4.4 hours |
| 99.99% | ~4.4 minutes | ~53 minutes |
Two things jump out. First, the gap between 99% and 99.9% is enormous — a full workday of outage per month is contractually fine at 99%. Second, every step past 99.9% is a step-change in engineering: redundant infrastructure, failover that actually fails over, and operational discipline. Treat any number ending in multiple nines as a claim that deserves follow-up questions, not applause.
(SIPSTACK’s committed number, for reference, is 99.95% — about 22 minutes of allowed downtime a month — published as a contractual SLA, not a website adjective. More on that distinction next.)
A number on a homepage is not an SLA
Section titled “A number on a homepage is not an SLA”The single most important question: is the uptime figure contractual? There are three tiers of “uptime number,” and they look identical in a sales deck:
- Marketing uptime. A percentage on the website with no definition, no measurement method, and no remedy. It’s a vibe.
- Historical uptime. “We achieved 99.98% last year.” Genuinely informative — but a record, not a promise.
- A Service Level Agreement. A document that defines what counts as downtime, how it’s measured, and what you’re owed when the commitment is missed. This is the only tier that means anything when things break.
If the number isn’t backed by a document you can read before you sign, it’s tier 1. Ask for the SLA. A provider that publishes one openly is telling you they expect to be held to it.
The fine print that decides everything
Section titled “The fine print that decides everything”Two SLAs with the same headline number can commit to wildly different things. When you read one, look for:
What counts as “down.” Total outage only? Or degraded service too — one-way audio, failed inbound calls, an unusable portal? The narrower the definition, the softer the promise. Phone systems have rich partial-failure modes; an SLA that only recognizes total collapse ignores most real incidents.
The measurement window. Monthly windows are the honest standard. An annual window lets a provider absorb one catastrophic day and still hit the number — 99.95% annually permits a single 4-hour outage with room to spare.
Exclusions. Every SLA excludes scheduled maintenance; read how much notice is required and when windows can occur. Watch for broader carve-outs — “carrier issues,” “force majeure” stretched to cover routine upstream failures, or problems “outside our network.” A phone service is a chain of carriers and networks; an SLA that excludes the chain excludes the product.
The remedy, and who has to claim it. Most SLAs pay service credits — a percentage of the monthly bill, scaled to the miss. Check whether credits are automatic or must be claimed within a deadline (almost always claimed — calendar it during any incident), and whether a chronic-failure clause lets you exit the contract without penalty after repeated misses. That termination right matters more than the credits: the real cost of downtime is your lost business, which no credit reimburses. The SLA’s job is alignment — making your pain cost the provider something — and an exit clause is the strongest form of alignment there is.
Five questions that sort providers quickly
Section titled “Five questions that sort providers quickly”- Is the uptime number contractual, and can we read the SLA before signing?
- What counts as downtime — and does degraded service count?
- What’s the measurement window, and where do we see reported status? (A public status page is a good transparency signal.)
- What are the exclusions, verbatim?
- If you miss the number repeatedly, can we leave without penalty?
Any provider serious about reliability answers these without friction — the answers are already written down. Evasion here predicts evasion during your first real outage.
Uptime numbers aren’t marketing decoration; they’re the one line of the contract you’ll reread during the worst hour of your quarter. Read it now instead, when you can still choose who’s on the other end of it.