Increasing number of logins on one extension

Our organization has multiple employees (about 9 employees) using the main line (extension 101) at different times of the day. We were told that there is a limit of only 3 devices logged in at one time... There are usually 3 or less people logged in at one time, but one phone is still getting a notification saying that too many devices are logged in and that it cannot receive incomings calls. It can only text or make outbound calls.

We also googled the amount of lines... And your website says "Up to 8 lines and 16 desk phones per group, with as many shared-lines groups as you need." Not sure if groups and lines are the same, but some clarification would be appreaciated.

We are hoping to purchase an add-on to a feature to increase the amount of logins so we won't run into the same issue.