Ok, so I always thought that the only way to clear a Fail to Communicate on a NX panel was to get it to actually communicate and get a proper kissoff from the receiver.
However, that is only true if you want the panel to communicate on account as it did before. As in an example the phone line is down for a couple of days. So it will continue to display the Fail to Comm. During that time frame you will need to enter the code twice to arm and disarm as was stated. Once the phone line is re-established then some signal needs to be generated and it will then clear the Fail to Comm once it does.
On the other hand, if the panel will no longer have a phone line and it is in a Fail to Comm mode it can be cleared by power cycling the panel. Then go into programming and turn off the dialer by placing zero in LOC 2. The Fail to Comm is cleared and the panel will not try to communicate thus all is good.
Now that may have been stated by others in one way or another. I just verified it for myself by testing it out on my bench to make sure.
Hope that helps someone.