Hi noscon,
Since we don't have access to the ISO15693 card, we cannot test them, but we can sure you that the T80 are able to read Mifare cards.
By the way, since there are already applications installed in the Nokias (business card, messages, ...), you are able to test tags without develop any MIDlet, but the T80 is shipped without default applications, but it at least vibrates if the tag can be read. If you said your T80 doesn't reacts, let's test it with Mifare tags to be sure it is not a failure of the device, like for instance, the antenna is not in contact with the pins after you removed it to remove the battery.
Best regards,