By eavesdropping on telephone calls, the investigators were aware who and when Mr Savisaar met and what to keep in mind while collecting evidence. The investigators were aware that dark matters are not discussed over the phone, and that plain speech would be avoided if at all possible.
Here is where hidden cameras are helpful, installed by court permission. However, even these are in danger to be discovered. Especially with Mr Savisaar, as the all-suspecting Mayor of Tallinn is in the habit of asking people close to him «bug control» places linked to him from time to time. Therefore, Kapo was at pains to diligently plan where and for how long to install stuff.