Several states including California, Florida, and New York tried to restrict the practice. Both Republicans and Democrats have sponsored federal legislation this year to limit charges for patients when hospital doctors are out of network.

Big money is at stake. Envision gets about $1 billion in annual revenue from UnitedHealthcare, the company says. Its total revenue last year was $7.8 billion. UnitedHealthcare said about 650,000 of its members received care from Envision clinicians last year.

Both companies have been trading blame for the hit patients will take if they don’t reach a deal over the terms of reimbursement, though both sides remain hopeful about reaching agreement. While stand-offs between insurers and medical providers are common, the stakes are especially high in this one because of the size of both companies and the fact that patients have little ability to avoid Envision's doctors in emergency situations.

The conflict has been heating up since October when UnitedHealthcare published a letter it sent to hospitals warning that an impasse could leave their patients dissatisfied “from higher out of pocket costs and patient confusion.” It also set up a website accusing Envision of “price gouging” with charges double the average for emergency services.

“We remain committed to working with Envision in finding a solution that will renew their participation in our network at rates that are affordable and predictable for the customers and members we serve,” UnitedHealthcare spokesman Stephen Shivinksy said in an email.

Envision blames insurers for gaps in coverage that leave patients exposed. "We’re just not understanding why United is interested in pushing a large emergency group out of network,” Bob Kneeley, a spokesperson for Envision, said.

Envision is one of a handful of big medical staffing companies that supply emergency doctors, anesthesiologists and other clinicians to hospitals nationwide. Physician-staffing companies can help lower costs for hospitals and bring doctors to areas where hospitals may struggle to find qualified staff.

The company was built through a series of acquisitions, culminating in a 2016 merger with AmSurg, a large surgery center and physician staffing group.  In October, private equity giant KKR purchased the combined company for $9.5 billion, including debt.

Envision bills independently from the hospitals where its doctors work. That arrangement means that even patients who choose a hospital in their health plan’s network may face charges from Envision's physicians who don’t take their insurance.

Since its merger with AmSurg, Envision has been trying to shake off a reputation for aggressive billing practices that it contends is undeserved.  AmSurg Chief Executive Officer Christopher Holden took over at Envision after the companies merged two years ago. He soon pledged to change course. “We are focused on moving the majority of our relationships to in-network status,” he told investors in February 2017, on his first earnings call leading the combined company.