ng-mask doesnt work conditional with expressionProperties

[hrishi1183]

ng-mask is not working conditional..
expressionProperties: {
"templateOptions.mask": "true ? '9999999999-99' : '(999)999-999'"
},
ex: one text field need to have different format based on its previous selection.. but expressionProperties is not working with ng-mask

http://jsbin.com/wixawijuca/1/edit?js,output

can anybody has any idea how to make ng-mask conditional