Talent Handicapper, Decision Analyst, GoTo Market Strategist, Sales Interviewer
The first thing I'd say is that nobody buys value propositions.
Value propositions are useful marketing tools, and useless sales tools.
In phone prospecting you have (approximately) 3 seconds to get 30 seconds to get 3 minutes to get 30 minutes.
If you don't sound right, you will have a challenge getting 3 seconds and rarely 30 seconds.
Your question is not actually about Value Proposition as much as it is about Prospecting.
This might be fixable...