If you may be scanning these devices in the future it's important to enter enough information to prevent asset duplication in the future. For network devices such as IP Phones the MAC address will be used as the unique key, so entering the MAC address when you first manually create the asset should prevent future duplication when they're eventually scanned.
Right now our security posture is preventing them from being scanned, so I am inputting the assets manually. I am not sure what Mitel would return as a value which is why I wanted to ask here before having to dig thru Mitel's OID tree.
I assume these Mitel systems don't have any protocols available to scan information from and are being retrieved as generic network devices. In this case, any additional information added to these devices would be manual, so the asset type will not make an impact. I'd recommend picking the asset type that most closely fits the device. You can also add additional asset types under Configuration\Asset Mapping.
If these systems are scannable through SNMP and are returning an OID you could remap this OID to a different asset type also under Configuration\Asset Mapping.