In addition to the difficulties many face in proving they have coverage, patients are also having a hard time figuring out whether particular doctors are affiliated with their health insurance plan. Doctors themselves often do not know if they are in the network of providers for plans sold on the exchange.

But interviews with doctors, hospital executives, pharmacists and newly insured people around the country suggest that the biggest challenge so far has been verifying coverage. A surge of enrollments in late December, just before the deadline for coverage to take effect, created backlogs at many state and federal exchanges and insurance companies in processing applications. As a result, many of those who enrolled have yet to receive an insurance card, policy number or bill.

Many are also having trouble reaching exchanges and insurance companies to confirm their enrollment or pay their first month’s premium. Doctors’ offices and pharmacies, too, are spending hours on the phone trying to verify patients’ coverage, sometimes to no avail.

“The system wasn’t really built to handle this kind of glut of new patients,” said Dr. Curtis Miyamoto, a radiation oncologist at Temple University Hospital who is president of the Philadelphia County Medical Society. “So it’s resulting in us having some delays in getting people verified, and therefore delays in their care.”