President Reagan's proposed 1985 changes in the tax code would not produce the same delays and computer failures that plagued the Internal Revenue Service in processing returns based on 1984 changes, IRS Commissioner Roscoe L. Egger Jr. told a skeptical Senate subcommittee yesterday.

Egger said Reagan's proposed legislation would result in "fewer changes than the 1984 tax act," thus making it easier to program the computers and monitor the results.

Sen. John Heinz (R-Pa.) said he found it "difficult to believe" that the changes would be "anything but more complex."

Delays in receiving new computers, computer failures and human errors in processing last year's tax returns kept taxpayers in Maryland and the District of Columbia from receiving refunds for 12 to 16 weeks after they submitted their returns, the longest "lag time" in at least five years, IRS officials have said.

Egger told the Senate Judiciary subcommittee on administrative practice and procedure that blame for the problems lies with the Sperry Corp., the telephone company and Congress.

Egger said the IRS is planning a 20 percent increase in telephone capacity, allowing the Philadelphia Service Center, where D.C. and Maryland returns are processed, to handle 20,000 to 25,000 calls per day. Last year, taxpayers placed twice that many calls to the Philadelphia center on an average day, phone company officials reportedly have said. Many calls went unanswered at the center, which has 53 phone lines.

"We planned on the basis of a normal season plus an additional factor," Egger said. "If we have the kind of crunch that we had last year, the phone company and the IRS can't handle it."

Sen. Arlen Specter (R-Pa.) replied, "The phone company can handle it if you have the phones."

Earlier in the hearing, Specter asked Egger how he responded this year when he heard about "complaints in superabundance" by taxpayers who constantly received busy signals when they called the Philadelphia office.

Egger said he "sent memos to everyone to respond to the telephone problem and do everything they can." He also said he asked the company for more lines but was told that it would take "several months to get the phone lines in."

Specter retorted, "I went to the seashore for two weeks and had a phone installed in two days."

Egger and Thomas Laycock, assistant IRS commissioner for computer services, blamed Sperry, a computer company, for delays in processing returns. Laycock said Sperry was 41 days late in delivering the last of its computers to the Philadelphia office.

Finally, Egger blamed Congress for the delays. "For a period of 10 years or more this agency went along on less money than we needed to provide additional service," Egger said.