The signal jamming in the National Assembly ahead of the state-of-the-nation-address was caused by an operational error, the state security department said on Wednesday.
"The signal disruption was caused by an operational error by the member on duty," the department said in statement.
"The operator failed to properly terminate the device and this impacted on proper access to some users of mobile phones."
The State Security Agency said it was responsible for threat and risk assessment, and on the day was supporting SA National Defence Force efforts to enforce airspace security.
On Thursday, journalists and some MPs protested against cellphone signals being blocked in Parliament.
Journalists protested in the press gallery because they could not file their stories.
"Bring back the signal, bring back the signal," they chanted, waving their cellphones at a black box which they believed was a jamming device.
The signal was restored after national assembly Speaker Baleka Mbete asked secretary to Parliament Gengezi Mgidlana to look into the matter.
The department said there was no executive or political decision to interfere with the free flow of information, it said.
"The Minister [David Mahlobo] responsible for State Security was also taken aback."
An investigation was underway with a possibility of disciplinary action for those responsible.
"This operation was never intended to frustrate the parliamentarians, the media and anyone who attended Parliament and the result of it not being switched off properly is highly regretted."
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