Communicated with support.
The reason is because Shoretel phones do not support SNMP. They were identified as "Shoretel, Inc" because of the MAC address manufacturer prefix. Although there is no way to automatically map them to an asset type, support gave me the following SQL script which correctly re-categorized the devices as "VOIP Phone":
UPDATE tblassets
SET assettype = 10, assettypelock = 1
WHERE EXISTS (SELECT 1
FROM tblassetcustom AS cust
WHERE cust.assetid = tblassets.assetid
AND cust.manufacturer like '%shoretel%')
It worked for me, but use at your own risk.
~ MiJaMu