After five wrenching months of uncertainty, the phone rang yesterday at the eastern Pennsylvania home of Alice and Harry Metrinko, bringing a message from the International Committee of the Red Cross that their son is alive.

"We're very, very happy," said Alice Metrinko, whose son has been described as the "forgotten" hostage in Tehran. "It's been a long ordeal." The Metrinkos, whose lonely vigil has drawn nationwide attention, were among the families who received phone calls yesterday from the ICRC. Most of the calls were made by Red Cross officials in New York, but the Metrinkos said the call to their home came from Tehran -- an indication of its urgency.

Embassy political officer Michael J. Metrinko, 33, is the only hostage whose family has received no letters since the takeover Nov. 4. Metrinko was not seen by American clergymen who visited the embassy on Christmas and Easter. He has never appeared in television film taken at the embassy.

The Metrinkos' worries were slightly eased last month after they talked with President Carter at the White House. Carter assured them that their son had been by an Iranian doctor and was apparently healthy, friends said afterward. But their fears persisted, partly because no neutral non-Iranian official had reported seeing their son.

Iranian militants were once offered $10,000 for a photo, message or other evidence that Metrinko was alive. "We still got no reply," said Rev. John Adams, who was among those making the offer.

Yesterday the doubts, worries and widespread rumors about Metrinko's whereabouts appeared to have ended, further confirming the State Department's expression of confidence that all 50 American hostages were seen by representatives of the ICRC.

"We are confident there are 50 people held hostage in the embassy," department spokesman Hodding Carter said yesterday after Monday's visit to the embassy compound by two International Red Cross representatives. The Red Cross officials had reported seeing all the hostages, thought they refused to say how many captives were in the embassy compound.

Perhaps nowhere in America was the sense of relief greater yesterday than at the Metrinkos' home in Olyphant, a working-class community near Scranton. About 11 a.m., the Metrinkos received an overseas call from Harold Schmid de Grueneck, the premanent representative of the International Red Cross in Tehran.

"They talked to our son. He was examined by a doctor. He's in very good condition -- very good spirits," Alice Metrinko said after the phone conversation. The Red Cross official passed on a brief message -- "Hi, Mom and Dad, I'm fine" -- and told the Metrinkos that a letter from their son was on the way.

Other hostages' relatives also expressed relief yesterday, including the family of at least one captive singled out by Iranian militants for denunciation as an alleged CIA agent.

The family of embassy political official William J. Daugherty, 32, received a message from the International Red Cross that he "was okay," Daughterty's brother, Charles, said in a telephone interview last night. Daugherty had been accused by Iranian militants of being a CIA agent posing under diplomatic cover. his brother's health or of the conditions under which he is being held. But he described the Red Cross report as "good news," partly because it was the first message the family had received from his brother since a Christmas letter. "There was a reasonable chance he was in Siberia for all we knew," he remarked.

Troy Kirtley also said he was pleased to hear from a Red Cross official, noting that he previously had received no word from his son since Christmastime. "At least I know the boy's alive and he's getting along all right," said Kirtley, whose son, Marine Lance Cpl. Steven W. Kirtley, 21, is a hostage. "That's something to be happy for.'

A message received by Dorothea Morefield, the wife of a hostage, appeared typical of those relayed by International Red Cross officials to the captives' families yesterday.

"Visited today by two Red Cross representatives," her husband, consul general Richard M. Morefield, 50, said in the brief note. "Have been receiving your letters with a three-to-four-week delay. I am still okay and in good health. I am doing lots of reading and I am studying Spanish and algebra to keep busy. I love you and will keep writing as often as I can. Give everyone my love."