They built telecom infrastructure already in the 1970s, the Nokia Mobile Phones company was created as an independent arm only in ~1989.
(which by no coincidence is the era the GSM standard was created: A global cellular telephony standard allowing interoperability across countries and suppliers)
Nokia's rise started in 1990 when Ollila, who had been the mobile phone lead, was promoted to CEO. This was still very early on (Nokia's first GSM phone only came out in 1992) and a large part of the fief war was driven by the legacy telco people being unhappy about the upstart mobile people now calling the shots.
The point is that Nokia was in the telecom sector for DECADES before the Microsoft acquisition of the Handset Division you mentioned.
> "and a large part of the fief war was driven by the legacy telco people being unhappy about the upstart mobile people now calling the shots."
Citation needed. Equally I have no idea what that has to do with my initial point that the device business was used to drive the infrastructure business.