The number of Lines you choose is the number of concurrent calls you can have active at once, incoming and outgoing.
Lines on MaxoTel services are a pool that can be used by all extensions.
The number of lines available for use is dictated by the plan you're on, and is not dependent on how many phone numbers (DIDs) you have on your account. For example, you could have a 4-Line plan with a single DID and still be able to utilise all four lines.
Internal calls between extensions on your service do not consume a line.
Any calls in from or out to external numbers consume a line as soon as the call is established, regardless of whether it's answered or ringing.
As an example, consider the following 4 line service usage:
In this example, we have 3 incoming calls and one outgoing call in progress (4 total), along with two internal calls between extensions. As the internal calls don't utilise a line, there can be 4 calls to or from external numbers, as many internal calls as required, and any additional calls in or out will receive a busy notification.