FORTUNE — GrubHub, the web and mobile-based food ordering firm, put in a lot of work to get to where it is today. It’s worth about $2.7 billion after its recent initial public offering, and serves 3.4 million diners at more than 28,000 restaurants after an August 2013 merger with New York-based Seamless.

Now comes the hard part: Getting restaurants to give up their seemingly indispensable fax machines and pick up a tablet computer.

Tucked inside GrubHub’s filings with the Securities and Exchange Commission for its April 4 I.P.O. is the fact that most of the restaurants it partners with get their orders sent to them by e-mail or, in many cases, fax. After that, an eatery receives an automated phone call, asking it to confirm the order number on that e-mail or fax. If that call doesn’t go through or screws up, a GrubHub customer service representative calls to straighten out the order. Those faxes and calls slow down the ordering and fulfillment process, negating the service that GrubHub provides.

GrubHub has a solution: Its OrderHub app pre-loaded on a customized Amazon Kindle Fire tablet that GrubHub gives the restaurant for free. It’s a relatively cheap expense for GrubHub, and provides better feedback to its customers. The device also cuts order confirmation times by 80 percent, and kills 85 percent of follow-up “Where’s my food” calls, according to the company’s own research.

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GrubHub, which took in $137 million in revenue in 2013 (up significantly from 2012), claims in its I.P.O. filing to have “several thousand” restaurants signed up, but that’s still a minority. Why haven’t more restaurants taken GrubHub up on its offer to modernize their order systems, speed up fulfillment, get employees off the phone, and increase their web-based revenues? A few reasons, some harder to tackle than others.

“We do it the old-fashioned way, with the fax, the safest way,” said Gus Aigy, manager of Bagels on the Ave in Astoria, N.Y. “[GrubHub] told me I could have [a tablet] if I wanted, but I stick with the fax and the phone call.”

Aigy gets “a good percentage” of orders from GrubHub, and he prefers online ordering to straight phone orders, where mistakes can happen on either end of the line during rush hours or loud periods. So he might consider GrubHub’s OrderHub tablet. “Maybe if they paid me,” he said with a shrug.

At Osteria 166 in Buffalo, N.Y., GrubHub orders arrive as e-mails. Those e-mails trigger an alert on owner Nick Pitillo’s phone, as well as the phones of his bar and kitchen managers. In this smaller city, online orders are a slow but growing part of his business. “It was a trickle at first, but now it’s picking up a bit,” Pitillo said. “Three this past Saturday, in fact.”

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Pitillo believes his restaurant could work with a tablet-based ordering system, but he said he needs more information. How would it integrate with his order-tracking point-of-sale system, for example? Point-of-sale systems from MICROS, Restaurant Pro, and others are one of the major challenges to GrubHub’s push for efficient online ordering, says Anand Sanwal, CEO and co-founder of the market research firm CB Insights.

“There are a lot of incumbent and emerging players in the POS market,” Sanwal told Fortune. “That said, GrubHub already has a sales force and a relationship with many restaurants. In a business like GrubHub’s, the scale they’ve built does offer them a unique and material advantage.”

GrubHub, which declined to comment while in its I.P.O. quiet period last week, told VentureBeat that its surveys show that 80% to 92% of restaurants have access to the Internet. But that access could very well mean a fax machine and nothing more. The food services industry is hardly alone: In law, health care, and finance, fax machines remain in use, despite their obsolescence.

For restaurants, there is good reason to keep a fax machine in use. If a restaurant uses paper order slips for cooks, it may seem redundant or troublesome to print out online orders. Some restaurants send weekly specials and menus out to nearby offices by fax. Others take group orders with the machine.

So long as there are customers that want to order with a fax machine, restaurants will keep one around — that is, unless GrubHub can do something about it.