Hello,
Adding to my above comment, There is also a bug with ALG feature which causes ALG not to function as expected and probably it could be the reason that when you enable SIP ALG then you are not able to call your extension.
The bug that i am referring to is as below:-
https://prsearch.juniper.net/InfoCenter/index?page=prcontent&id=PR1120757
Thanks,
Pulkit Bhandari