By Rory Cellan-Jones
Technology correspondent, BBC News
Mobiles and the net are hugely popular among young Iranians
As protests continue in Iran, details are emerging of the technology used to monitor its citizens.
Iran is well known for filtering the net, but the government has moved to do the same for mobile phones.
Nokia Siemens Network has confirmed it supplied Iran with the technology needed to monitor, control, and read local telephone calls.
It told the BBC that it sold a product called the Monitoring Centre to Iran Telecom in the second half of 2008.
Nokia Siemens, a joint venture between the Finnish and German companies, supplied the system to Iran through its Intelligent Solutions business, which was sold in March 2009 to Perusa Partners Fund 1LP, a German investment firm.
The product allows authorities to monitor any communications across a network, including voice calls, text messaging, instant messages, and web traffic.
But Nokia Siemens says the product is only being used, in Iran, for the monitoring of local telephone calls on fixed and mobile lines.
Rather than just block traffic, it is understood that the monitoring system can also interrogate data to see what information is being passed back and forth.
A spokesman described the system as "a standard architecture that the world's governments use for lawful intercept".
He added: "Western governments, including the UK, don't allow you to build networks without having this functionality."
Asked by the BBC about the firm's attitude to doing business in Iran, Nokia Siemens said: "We do have a choice about doing business there, and on balance providing connectivity means there is a net benefit."
He explained that millions of Iranians were getting mobile phone services through Nokia. "The amount of information that is coming out of Iran from ordinary users because they have connectivity that they would not have had before is of a net benefit to them."
"I don't think Iran would have expanded its mobile network and its connectivity to its citizens if it had not had this capability."
Nokia Siemens markets the Monitoring Centre product to 150 countries around the world where it does business. The firm says it does not supply the system to China or to Burma.
The phone monitoring system sits side-by-side with the extensive net filtering system Iran has constructed in recent years.
Traffic in and out of Iran is largely controlled by Iran Telecom. On 13 June, the day after presidential elections, data traffic come to an almost complete halt, according to analysis by network security firm Arbor Networks.
Since then, traffic has gradually recovered, and analysts have speculated that the slowdown and re-start was caused by authorities putting in place filtering and monitoring systems.
Because Iran is effectively reading every message, this results in an inevitable slow down of traffic.
In mid-June, the OpenNet Initiative, which surveys net-watching efforts, updated its survey of net use in Iran and said the nation was: "investing in improving its technical capacity to extensively monitor the behavior of its citizens on the internet."
It said women's rights activists arrested in the nation had been shown transcripts of instant messages they had sent.
"If true," said the survey, the evidence, "would support the existence of an advanced surveillance program."