You must obtain service from an Internet telephony service provider, or ITSP in industry jargon. They are commonly called VoIP providers in this community. Just an ISP is not enough when you want to call the “real world” such as cell phones. I don’t know about specific providers but voip-info.org should be a good starting point for a search.
Your provider must allow you to have more than one concurrent conversations if you want to forward calls programmatically from Asterisk. You probably also need to pay for more than one “phone number” (sometimes known as DID numbers). This is not strictly necessary if all you need is what you described, but in order to give your more than one concurrent calls, a provider may want to bundle one number for each concurrent call.
These items are technical drivers behind price. Package is just a marketing gimmick on top of these. You need to examine your own needs first. (Read about VoIP services and providers will help you identify your needs.) Then, when someone throws you a package, you know what to ask.