Do both i.e. charge them $1 for 1000 calls, and cap it at $30 per month for a period of 3 months - after which they would need to sign up for the monthly plan t

Do both i.e. charge them $1 for 1000 calls, and cap it at $30 per month for a period of 3 months - after which they would need to sign up for the monthly plan to continue to get the $30 rate.