The companies include Thomson Directories and residential phone company Toucan Telecom. The other five are telemarketing specialists Promote IT, The Listening Company, ANT Marketing, Fax Information Services and Firestorm Marketing.
The companies were identified by public complaints made directly to Ofcom and to BT's nuisance calls bureau.
Ofcom has previously taken action on silent calls in its investigation into the volume of silent calls made by Kitchens Direct. It required the company to make a written undertaking that silent calls should account for no more than 5% of its call volume, and has threatened a maximum fine of £5,000 if it breaches the undertaking.
In addition to the silent calls caused by operators not being immediately available to take calls made by automated dialing equipment, the investigation covers calls made by fax machines contacting phones without the capability to receive faxes.
A Thomson Directories spokeswoman confirmed it had received notification of the investigation from Ofcom, but said it was too early to comment on any circumstances that might have led to it being contacted.
The investigation follows last week's decision by the DMA to toughen its code of practice on telemarketing.
Pic: Getty Images
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