You can always add individual features to your OnSIP phone system. It doesn't matter if you're on the Pay As You Go plan, Basic plan, or Per Seat Unlimited plan. If you would like to add a feature that is currently not included in your plan, simply log into the Admin Portal and set up features through the "Apps" or "Resources" tabs.
Let's say your business is on the Per Seat Unlimited Plan. You have 15 users each with their own personal voicemail boxes but you need another one for your support ACD Queue. To add another voicemail box, simply create a new one through the "Resources" tab.
You can see this extra resource added to your future invoice by going to the "Account" tab and clicking on the "Configure plan" link. In the "Configure Plan" box, you will see your new voicemail box under Additional Services, and the monthly total that you owe will increase accordingly. Any additional features are priced as pay-as-you-go features.