You'd need guts to take the reins at Nokia when it was stumbling into possible bankruptcy-- but imagine phoning Steve Ballmer at home and demanding 2 billion.

That's just what Nokia Chairman Risto Siilasmaa did while negotiating the sale of the company's phone division to Microsoft back in 2013 with the software giant's then-CEO.

Sillasmaa was interim CEO at the time, and now acts as chairman; he also founded Finnish security firm F-Secure. Speaking at Nexterday North in Helsinki, he proudly told attendees that Nokia's enterprise value is now 15 times what it was three and a half years ago.

"It's quite rare and unique to see such a deep transformation in such a short time," he said.

At the time Nokia and Microsoft were negotiating terms of the 4.6bn deal, Siilasmaa was also in talks to buy the remainder of Nokia Siemens Networks (NSN), but Nokia didn't have the cash and he couldn't get investors to help. "We couldn't even get meetings with these investors," he said, calling it a "very tough time emotionally".

Saying trust was the key to any negotiations, he said: "In the Microsoft negotiations, we had four moments when the negotiations broke down. And we always recovered, based on the trust that had been created."