The phone can potentially not display all lines if the incorrect Phone Button Template is assigned to it.
In order to fix this, choose the phone configuration window, scroll down to Phone Button Template, and click the link next to it.
This opens a new page that shows how the buttons are configured. It is possible to go to Device > Device Settings > Phone Button Template in order to verify that the settings are correct. If needed, create a new template, then assign it to the phone.
By default, the Phone Button Template contains two lines and four speed dials. More lines can be added to the IP phone with either a new Phone Button Template or a modification to the existing Phone Button Template. All combinations of lines and speed dials, such as one line, five speed dials and two lines, four speed dials, can be configured. A maximum of six lines can be configured.
In order to create or modify a Phone Button Template, complete these steps:
Make a copy of the existing Phone Button Template and re-name it.
Modify the new template to reflect the number of required lines and the number of required speed dials.
On the phone configuration page, assign this template to the phone.