First, telemarketing is a probability, which means that you have to make as many calls as possible to find more customers.
The second is to serve customers well after discovering them. You can't say that the customer finished the task as soon as he took it. Try to make the customer feel dependent on you.
Third, let customers introduce you to new customers, which is also the most important sales skill in telemarketing. Introduce, introduce customers, customers feel completely different from your fully developed new customers. At least customers have some trust in you, and there is not much strangeness. All telemarketing is getting bigger and bigger through recommendation.
Telemarketing is difficult. Good luck.