We can only fetch emails & phone numbers from the profiles which share these data with you (usually, 1st connection – the ones you connected).
You might see this notice when you click on the profile name in the Linked Booster dashboard.
According to LinkedIn’s hard rule, you usually don’t share your contact information yourself to 2nd and 3rd connections, unless you make your information public, such as the profile below.
So Linked Booster will help you to collect available contact information shared with you whether it is first, second or third connection. You can use this feature to visit up to 100 profiles per day.