Yep! Your contacts will be auto-imported from calls after an automatic opt-in message is sent to ensure the contact wants to receive your messages (don't worry, you can customize this opt-in message to your liking).
As soon as you are ready to send a campaign, the total active contacts will be confirmed and the message will be sent. Contacts that are permanently stored in the SMSMMS Campaigns area of dbaPlatform can be manually removed, and the cost will go down.
To see a reduction in your bill, the contacts count must be reduced for one full billing cycle leading up to your charge date. If you remove contacts in the middle of a billing cycle, you won't see a reduction in charges until after the next full billing cycle.