The best solution would be IP Office and leverage Avaya Workplace soft phone on Desktops, and Mobile devices.
This way the reception can see the presence status and also message the CEO without the need to call.
If the business if open to cloud, configuring the messaging engine to use Avaya Spaces is a great option, as it allows messaging off net as well. The remote part just needs a free spaces account, and once you send a direct message from with Spaces Web Portal to the remote party, that remote party will also show up in Avaya Workplaces as well. From that point on, messaging with remote parties can happen from within the soft phone.
If the organization is not open to cloud, then configure IP Office to use One-X Portal as the messaging engine. This will in internal messaging only.
With all the focus on cloud, I would strongly recommend the IP Office with Avaya Spaces approach as it probably going have a longer life span than One-X Portal.