"Don't talk to the foreigners," said the voice. We were interviewing a leading Shia dissident in his home when he was interrupted by the call.
It was a state security man who had tailed us to the house and was now parked outside.
The dissident, Dr Tawfik Alsaif, laughed and put down the phone. "They don't realise how things have changed yet," he said.
We were in the eastern town of Qatif in the heartland of Saudi Arabia's minority Shias and the scene of small anti-government marches last week.
The demonstrations were mainly by families demanding that their relatives be released from prison.
On Friday, activists are trying to organise a Day of Rage in cities across the kingdom.
But calls for political reform are now coming from different sections of Saudi society - Shia and Sunni, conservative and liberal. It is this that alarms the authorities.
Dr Alsaif, a Shia intellectual, has made common cause with other campaigners.
He had signed two of the three petitions circulating on the internet calling for more democracy in the kingdom.
There was a time when talking to the BBC would have got him arrested, and perhaps it yet will.
But, he told me, after Tunisia, Egypt, Bahrain and Libya, there was a new paradigm in the Middle East, and Saudi Arabia could not be immune.
"Every state in the Middle East is nervous. And they should be nervous," he said.
He stressed that he wanted gradual change, not a revolution.
"I don't believe that liberal democracy will be put in place tomorrow but we have to start somewhere. Equality, the rule of law - the country is ready for this. We have to start the process."
After that meeting, the authorities asked us politely, but firmly, to leave eastern Saudi Arabia.
Before we were put on the next plane back to the capital, our government minders found us some loyalists to speak to.
They worried that democracy would cause strife in a tribal society.
"Stability and security are very important here on the peninsula," said Adeeb al-Khunaizi, an oil executive from a family of Qatif notables.
Democracy risked "60 years of bloodshed", he said - a return to the era of tribal conflict.
Salman al-Jishi, a senior businessman, said that people loved the king and democracy was not needed in a country where anyone could take up their grievances directly with their rulers.
"The doors are open here," he said. "You can pick up the phone and call a prince and he is willing to hear you and listen to your problems. We have a different system. I don't think that even in your country you can meet any minister [you want]."
This is SAUDI Arabia - a country named after one family, the al-Saud dynasty.
There is no serious challenge yet to King Abdullah's rule. Still, on his return from medical treatment abroad, he announced £23bn ($37bn) in pay rises, subsidies and gifts for his people.
That was the carrot. The interior ministry has produced the stick, issuing a statement to remind Saudis that public demonstrations in the kingdom are illegal, warning that "all measures" would be taken to deal with "disorder".
The ministry's spokesman, Maj Gen Mansour al-Turki, told me that the security forces would move very quickly to close down any demonstrations before they could escalate.
"There is no order to shoot. There is no order to use any kind of excessive force," he said.
"Our people are very well trained in dealing with demonstrations. They are used to this from the Haj [the annual pilgrimage to Mecca which attracts hundreds of thousands of people]."
Fouad al-Farhan, a blogger who has served time in prison, told me: "Demonstrations are not part of the culture. The majority of Saudis never saw a protest. That is a barrier. Will people go out? That is a big question."
That question was being debated at a dinner in Riyadh for another internet activist who had only just got out of jail. His crime, he told me, was that he had discussed the royal succession on his Facebook page.
We were joined by Mohammed Qatani, of the independent Saudi Civil and Political Rights Association.
He said there were 30,000 political prisoners in the kingdom, some of whom had been held for years without trial. The authorities say that people are jailed for security reasons.
"People are glued to their TVs, watching and learning," he said. "I think the momentum is building."
The authorities had to give some signals about reform now. "Things could escalate quite quickly and then by the time you make concessions, it is going to be too late."
Men sat on the floor, eating, Bedouin-style, with their hands from vast trays of lamb and rice.
Even here, in a gathering of human rights activists, most said they would probably not go out onto the streets tomorrow.
Political debate in Saudi Arabia is still something carried out in private, in people's homes, or anonymously, on the internet. You cannot hold rallies and street protest is illegal, as are political parties.
Given all that, any kind of gathering on Friday - however small - would send tremors through the government.
Neither the authorities nor the activists know exactly what to expect - but probably not a repeat of what has happened in Egypt, Tunisia or Libya.
Or, as blogger Fouad al-Farhan told me: "Saudi Arabia is not like other countries - where people have nothing to lose. Here, they will be cautious. So many Saudis still have something they don't want to lose."