There was a teenager cursing on the internet. That wouldn’t be unusual, except it was 1993.

Dane Jasper and Scott Doty were students at Santa Rosa Junior College, and one of their jobs was setting up dial-up internet service at the college. In the early 1990s, internet access was becoming available on college campuses, but not everyone had it at home.

That’s when they got a call about Max posting profanity using his college account — and that’s what led them down the path of starting Sonoma Interconnect, now known as Sonic, the largest independent internet service provider in Northern California.

Jasper and Doty couldn’t find a student by that name when they looked him up. The student ID number on the account belonged to a 78-year-old woman named Mildred — someone who seemed unlikely to be spending her time using foul language online.

The pair eventually figured out the scheme. Students working in registration were stealing the student ID numbers of older adults signing up for noncomputer classes — the credentials needed to get online — and selling them to high schoolers eager to get on the internet.

“I said, ‘Scott, people are stealing this from us. We should set up a private service that provides access to the internet,’” Jasper said.

But where would they get the money to start Sonic? Doty had an answer: He opened up a drawer with paychecks from his campus job and remarked that payroll had been asking him to cash them.

The very notion of internet service providers and dial-up may seem like a legacy of the 1990s, which makes Sonic’s survival and growth all the more remarkable. The internet company marked its 25th anniversary Friday — a quarter-century after Jasper and Doty registered the domain Sonic.net.

What started in the back room of Jasper’s mother’s house has grown to 500 employees serving more than 100,000 customers in California.

In the early days of the company, Jasper and Doty ran classified ads in the newspaper to sign people up; customers mailed checks for three months of service; and the pair would set up internet connections in the evenings and on the weekends.

“This was a time when people were starting to get modem lines and fax lines, so there was a real demand for second phone lines in people’s homes,” Jasper said. “I put about 20 phone lines in my mom’s house, and I put in an order for eight more phone lines and I got a call from the engineer at Pacific Bell, indicating that they couldn’t put in all these additional phone lines into this residential area.”

The phone company eventually relented — in what would be the first of Jasper’s many disputes with the telecom establishment — but capped the extra lines at eight. Sonic eventually moved into a 95-square-foot office in downtown Santa Rosa. The office building had about 300 phone lines, which Sonic quickly used up as it continued to expand.

“As the internet grew, the business grew,” Jasper said. “So we had very lucky timing and were at the right place at the right time with the right skills and a little flash of insight that this was valuable.”

Jasper thinks of Sonic’s history in four phases. First there was dial-up internet — the modem screech that anyone who went online in the 1990s remembers. The DSL era followed, as technology progressed and those same copper lines that carried dial-up signals were reused for higher-speed connections. The third phase was becoming a full-scale phone company offering broadband and voice calling in 2010. That was what really “catapulted” Sonic’s growth, Jasper said, as it meant Sonic was deploying more of its own equipment and providing what was an innovative product at the time. Now, the business is in its fourth phase: building its own gigabit fiber network in California.

Sonic’s fiber offering, which competes with high-speed internet service from the likes of Comcast and AT&T, was ranked the fastest internet service provider in the country last month by PCMag.com. The company announced its largest expansion of the service yet in March, when it expanded to 19 new neighborhoods or cities in the Bay Area. More than a third of Sonic’s customers are on its gigabit fiber service, and the company is moving fast to deploy more fiber.

For an independent internet provider like Sonic to stick around and keep expanding for 25 years is rare. The number of independent providers competing in California was at its peak around 1997, according to Peggy Dolgenos, the co-CEO of Cruzio Internet in Santa Cruz. There were around 400 independent providers competing in the state in 1997. That’s now down to about a dozen, said Dolgenos, who is also a board member of the California Internet Solution Provider Association.

Dolgenos, who called Cruzio and Sonic “BFFs,” or “best friends forever,” said the independents have had to provide better service to compete with giant corporations and survive.

Jasper wouldn’t share current revenue for the privately held company. He told The Chronicle last year that Sonic turned a profit after three months and hadn’t had an unprofitable quarter since.

Sonic’s survival is remarkable, observers agreed. With internet service, economics favor natural monopolies, according to Abhishek Nagaraj, an assistant professor at UC Berkeley’s Haas School of Business. Because it’s expensive to start up service, the market tends to naturally concentrate to a few players.

“Regional ISPs tend to be more flexible, more innovative,” Nagaraj said. “They can more quickly cater to the needs of the local market. They tend to also serve underserved communities and markets that are harder to reach. The economics for them make sense.”

Part of Sonic’s popularity can be attributed to its product quality, such as its gigabit fiber, customer service and people’s tendency to root for the smaller player, Nagaraj said.

“I think some of it purely because they are an alternative to the largest players like AT&T and Comcast,” Nagaraj said. “They’re just more responsive to customer needs.”

Indeed, it’s easy to find the email address of Eli Caul, director of customer care, on Sonic’s customer forums, and he signs network status updates that bigger firms would post anonymously.

“Most businesses to this day, they see the contact center as a cost center that needs to be mitigated, whereas here at Sonic we see customer service as our primary service differentiator,” Caul said.

Caul joined Sonic in October 1996 and is its longest-tenured employee. He implemented a system where Sonic employees called new users after a week to make sure everything was going smoothly and the customers knew how to reach Sonic.

“People really do appreciate that,” Caul said. “Being able to pick up the phone and talk to a human being who can really respond to what you need instead of just checking the box.”

Jasper’s job has changed from the days when he personally configured customers’ modems. He’s finding himself on planes to Washington, lobbying for customers’ privacy and equal access to telecom infrastructure.

“Somebody who’s able to hang in there and compete against some of the largest companies in the country with massive lobbying dollars ... it’s very hard to do business in that environment,” Dolgenos of Cruzio said. “We do things like protect net neutrality ... we do much more to protect our users’ privacy, we’re not selling their data.”

Does Sonic have a fifth phase ahead? Jasper was coy but hinted that it might lie even farther from Sonoma County.

“We’re working very hard to connect as many homes as we can to fiber as quickly as we can,” Jasper said. “We have work to do locally here in the San Francisco Bay Area to connect more of those folks, but this is not a market or a business model that is unique to the Bay Area, or a need that’s unique to the San Francisco Bay Area, it’s nationwide. It needs to be addressed. I see it as an opportunity.”

Sophia Kunthara is a San Francisco Chronicle staff writer. Email: sophia.kunthara@sfchronicle.com Twitter: @SophiaKunthara