Source: anyone who actually follows the industry. Eriksson has been one of the leaders of mobile networking gear since the times of NMT. It has also been the leading phone manufacturer back in the days of NMT and GSM.
It hasn't been a major phone manufacturer since early 2000s however, when its mobile phone unit got crushed by Nokia. That's why they had to do the join venture with Sony.
When that mostly fell through, they sold the unit to Sony, while getting to keep most of the patents and licensing them to the Sony's mobile phone division. This is EXACTLY what happened to Nokia and Microsoft - Nokia kept most of the patents and licensed them to Microsoft.
The reason why this deal is so good is because buying company saves a lot of money on not purchasing the patents but getting to use them. Meanwhile selling company gets to go very aggressive on mobile phone patent violation claims because it no longer has a phone production unit that needs to be shielded from counter suits.
That is why when Nokia-Microsoft deal started to be made, a lot of regulators sought assurances that Nokia wouldn't go full patent troll with its mobile phone related patents after it has no phone manufacturing business of its own to risk.