There is no non-ringing option for a 500 set in the CLS. You would have to turn off or disconnect the ringer.
The CS1000 was interpreting your MCNA as NAMA, a totally different feature.
So why did it take MCNA?
Back in the day, many of the commands were created so that they could be changed using the dialpad of a phone or attendant console. So entering M or N is the same as the digit 6 on the dialpad. C and A are equivalent of digit 1. This doesn't always apply everywhere now but it's still a thing, especially with LD 10, 11 or 32.
Example - when logging in, the command is "supposed" to be
logi <userid>
But we can use
loii <userid>
or even
5644 <userid>
This also explains why the * is so important in CS1000 commands.