It looks good! Now users can use the same feature codes for a dahdi channel and for selintra...
I would have a comment (keep in mind I'm not a coder):
It could be interesting to have extension.conf using an include after the new lines:
;
; NANP Vertical Service Code Compatibility
;
...
exten => *98,1,agi(selintra,*51*)
exten => _*99XXXX,1,agi(selintra,*61*${EXTEN:3})
#include sark_customer_nanpfeatures.conf
Many thanks again for your cooperation with these features. They will be appreciated!