One of the biggest challenges my clients share is their teams struggle to get hold of their Prospects. My recommendation is book telephone appointments in theirs and your diary. This way they have to accept the appointment and then they expect your call. Your contact rate will go through the roof.
And if it does go to voice mail you say “John, calling as promised, you must be on the other line, so call me back or I’ll try you again in 15m”.
You’ve given yourself another reason to call back, as you’re following up like a professional, rather than chasing like a pest.