Most likely your modem doesn't detect Caller ID on legitimate calls from time to time. Please open PhoneTray, click Calls - All on the left navigation panel and check if you have calls with a blank number and name. If you do, please go to the Privacy - Call Blocking screen and uncheck "Block calls with empty caller's number" on the right panel:
That will prevent PhoneTray from blocking legitimate calls when your modem doesn't detect caller ID on the call.
If you have a large number of calls with a blank name and number, your modem cannot detect caller ID reliably and you will need to replace the modem. We have a list of recommended modems. New model Hiro H50228 (the second modem listed) has very reliable Caller ID detection. Please give it a try.
You can find out why the call was blocked by hovering your mouse over the blocked icon left to the blocked call on the Calls screen: