EVANS CITY, Pa. (AP) _ Sheila Nagy invited President Reagan to dinner, but when her phone rang and the operator said, ''Please hold for the president,'' the startled homemaker asked, ''The president of what?''

With first lady Nancy Reagan recuperating from breast cancer surgery, Reagan called Saturday afternoon to express regret that other commitments prevented him from accepting the invitation.

''But he said he and Nancy might take us up on it some other time,'' Mrs. Nagy said.

Reagan also chatted with Mrs. Nagy's husband, Lewis, a truck driver, and three of their five children.

Nagy, 42, said Sunday that 10 years ago he was laid off, and the family was forced to live on welfare benefits and his earnings from odd jobs. He credits Reagan for his changed circumstances.

''I'm working two jobs, driving a truck for Fort Pitt brand meats in Evans City and at Stanley's Donut Shop in Butler,'' he said.

Although his wages from Fort Pitt have declined slightly since Reagan took office, ''I'm getting more for my money,'' Nagy said.

White House assistant press secretary Liz Murphy confirmed that the call took place.

Mrs. Nagy said she did not expect a personal reply after she wrote Reagan Sept. 10 to thank him for improving the nation's economy and invited him to have dinner with her family. In her letter, Mrs. Nagy asked that she be warned ahead of time if he was going to stop by.

''I told him to please call in advance and let me know ... so I could take enough meat from the freezer and put a couple of extra potatoes in the pot,'' she said. ''I invited him to see what it was like living the normal American family life.''

Although the experience was exciting, talking to the president ''was like talking to the neighbor next door,'' Mrs. Nagy said. ''I couldn't believe that I was so relaxed.''