"Listen to this! One. Two. Three!" Down the crackling phone line from the women's wing of Tehran's Evin prison, a chorus of prisoners then launch into raucous song.

It's a Persian rendition of the Italian protest song "Bella Ciao".

"All for one and one for all!" they sing, laughing in shared defiance in support of the "Woman, life, freedom" protests that have shaken Iran's clerical authorities for five months.

The audio clip of the January telephone call, released on social media by a daughter of one of those held, has become a symbol of the courage of the women held in Evin prison and their refusal to stop campaigning even behind bars.

Many such as environmental activist Niloufar Bayani, arrested in 2018, have been held for several years. Others including the activist Narges Mohammadi, tipped by supporters as a Nobel Peace Prize contender, have spent much of the past decade in and out of jail.

Some were arrested well before the women-led protests sparked by the September 16 death in custody of Mahsa Amini, a young Iranian Kurd who had been detained for allegedly violating the strict dress code for women.

But their numbers swelled in the ensuing crackdown.

Several women have been released in recent weeks, including Alieh Motalebzadeh, a journalist and women's rights campaigner whose daughter posted the viral clip of the "Bella Ciao" protest song, and French-Iranian academic Fariba Adelkhah.

- 'Basic rights and freedoms' -

But campaigners have rejected the amnesty as a PR stunt and key figures remain detained.

They include Bayani and Mohammadi and also environmental campaigner Sepideh Kashani, arrested in the same case as Bayani, the labour activist Sepideh Gholian, journalist Golrokh Iraee, arrested in the protest crackdown, and German-Iranian Nahid Taghavi.

Also held in Evin are Fariba Kamalabadi and Mahvash Sabet, two members of the Bahai faith not recognised by the Islamic republic who were detained in July and are now serving a 10-year prison sentence apiece for the second time in their lives.

These women remain deprived of their freedom because Iran's clerical authorities under supreme leader Ayatollah Ali Khamenei "tremble at their words", said Jasmin Ramsey, deputy director of the New York-based Center for Human Rights in Iran (CHRI).

"The hijab headscarf is a pillar of the Islamic revolution and so is the subjugation of women. They hate it when women speak out and say 'I can do anything!'" she told AFP.

Ramsey dismissed the recent amnesty, saying: "The doors of Iranian prisons are revolving when it comes to political prisoners... The prisons will swell when there are more protests."

Of those who remain jailed, she said: "Many need medical help and their basic human rights have been violated for so many years."

The CHRI is now leading a petition signed by almost 40 other rights groups and directed at the current European Union presidency holder Sweden urging EU nations to summon Iranian ambassadors in unison for International Women's Day on March 8.

The ambassadors should be told to "stop detaining and committing violence against women who are calling for basic rights and freedoms in Iran" and to "end the physical and sexual violence against women detainees and protesters", it said.

- 'Sound of a revolution' -

Mohammadi, a member of the chorus in the "Bella Ciao" song, has in the last months emerged as among the most outspoken of those held, denouncing the conditions in Evin and vocally supporting the protests.

"Narges does not stay silent. This is not acceptable for the Iranian government," her Paris-based husband Taghi Rahmani told AFP in October.

In December, she released an open letter from prison denouncing the sexual assault of detainees and detailing shocking cases of women being raped by their interrogators.

"I believe that we, the brave, resilient, lively and hopeful women of Iran, will come to the streets and will continue to fight despite the government's repressive and violent measures and despite the danger of assault and even rape."

Sepideh Gholian, who is serving a five-year sentence on national security charges after supporting a strike by workers, in a lacerating letter published by BBC Persian in January described the methods used by interrogators to force confessions and the screams heard within the prison.

"Today the sounds we hear... across Iran are louder than the sounds in interrogation rooms; this is the sound of a revolution, the true sound of 'Woman, life, freedom'," she said.

The women have also launched appeals published on the Instagram account of Mohammadi for the Islamic republic to halt executions, after four men were hanged in cases related to the protests.

"The women have shown they are voices of change, freedom and equality. One reason Narges is still there is they (the authorities) are scared of her. She makes them quiver," said Ramsey.