Primary Dial-in Numbers will be displayed at the top of all invitations for scheduled calls. The first Primary Dial-In will be the one displayed in the grey bar at the bottom of every page in your account.
You can change and add Primary Dial-in Number/s for your account by logging in and clicking the Settings tab, then choosing Primary Dial-in Numbers in the left-side menu.
This will display a field where you can enter the country or city of your choice. Click on your preferred location and it will be added to the list. You can add as many Primary Dial-In Numbers as you like. Then click on Save when you're done to lock in your selection.