Sources said the scrutiny will take another two months after which CBI will be filing a regular case in the matter.
The agency had sought these documents from the state run telecom firm after filing preliminary enquiry in the case.
The sources in the agency said they were trying to figure out how the allocation of such a huge number of ISDN lines capable of carrying very large data was made to Chennai residence of the former Telecome Minister and how was it connected to Sun TV Channel office.
They are also trying to analyse the losses suffered by the operator because of these dedicated lines.
The agency had sought documents related to sanction given for setting these ISDN lines which are capable for carrying huge amount of data.
The CBI has asked the Telecommunication Department to provide them with technical details of the alleged set-up connecting Sun TV office and former Telecom Minister Dayanidhi Maran's residence.
Nearly four years after getting complaints, the CBI has started a formal probe into allegations that a 'virtual' telephone exchange was set-up at the then Telecom Minister's for facilitating data transfer from Sun TV owned by his brother Kalanithi Maran.
These 323 residential lines were in the name of BSNL General Manager connecting the
Boat House residence of Maran with office of Sun TV through a dedicated underground cable during the tenure of Maran as Telecom Minister, the sources claimed.
The agency recommended action in the case to Telecom Secretary in 2007 but the department allegedly did not give its nod, they said.
The CBI has alleged that these lines, used by medium to large commercial enterprises to meet special needs such as video conferencing and transmission of huge volume of digital data, for which a heavy price is to be paid, were almost free for Sun TV through this set-up.
It said that this virtual exchange was programmed in such a way that except "authorised BSNL staff", no one in the state-owned telecom company was aware of its existence.
The CBI had alleged that a stealth cable was used to connect the residence of the Minister with Sun TV office to give impression that these lines were for the Minister but actually they were used by the channel.