I encounted this myself too.
The only call forward I have enabled is when unavailable and it goes to a missed call alert service my provider has. I've only seen this happening with 1 parson in particular and with 2 numbers that that person has.
So when this person calls me, I do not see any alert like what you have experienced and instead, I get alerted to a missed call by my service provider. Everybody else who tries to call me gets through normally. On farther testing, my friend who couldn't get me before is connected normally only on the 3rd try.
It's really puzzling.