The sales are the key planks of a revamp plan put forward by Former TIM deputy general manager Stefano Siragusa and London-based investment firm Merlyn Advisors, representing shareholders owning a total of under 3% of TIM and challenging the phone group's project to sell its landline grid.

Under this alternative scheme, dubbed TimValue, TIM would retain its entire fixed network business as well cloud and digital services operations, while selling its Italian retail business and its prized Brazilian subsidiary to cut its heavy debts.

The group of investors would be ready to raise its shareholding to over 5%, if needed, in order to be able to call an extraordinary shareholders meeting to vote on the matter, Siragusa said in an interview with financial daily Il Sole 24 Ore.

He said that TIM's top investor Vivendi, which has raised major reservations over the sale of the fixed line network, was not "behind" TimValue.

"The real issue is to involve as many shareholders as possible in a project different from that proposed by management," he said, adding the plan was "solid" both financially and industrially.

The challenge plan comes after U.S. fund KKR made a binding offer for TIM's grid that valued it at around 23 billion euros, including debt and some variable components.

TIM directors will on Friday begin assessing the bid by KKR and Vivendi has called on the board to fully examine the TimValue proposal before any decision, a letter seen by Reuters showed.

($1 = 0.9436 euros)

(Reporting by Giulia Segreti, editing Federico Maccioni and Tomasz Janowski)