Sippy I had the same issue. I comment the line #include features_general_additional.conf from /etc/asterisk/features.conf and modified copy the line pickupexten=*8 from /etc/asterisk/features_general_additional.conf to /etc/asterisk/features_general_custom.conf, then reload module features and check with "features show" the change applied.
cat /etc/asterisk/features.conf
[general]
;#include features_general_additional.conf
#include features_general_custom.conf
[applicationmap]
#include features_applicationmap_additional.conf
#include features_applicationmap_custom.conf
[featuremap]
#include features_featuremap_additional.conf
#include features_featuremap_custom.conf
cat /etc/asterisk/features_general_custom.conf
pickupexten=*8