When customers reply to a shortcode make sure they are sending the reply without prefix '+'. It seems that when sending a message to shortcode 60006 (or any shortcode) with a + prefixed to it, the message will not be sent. Technical support tested this on quite a few phones and some older models are adding a + prefix when replying back to a shortcode. If it does, the user should remove the + before sending otherwise the message won't be received.
On a similar note, if a 0 prefix is added like 060006, it is possible the message MAY still be delivered (network dependent). We have a case where this was done on a FREETEXT shortcode, the message was delivered but the person was charged.