Format telephone number binding

Question

How can I format a telephone number like the following using knockout?

<a href="tel:12345679">
    call us
</a>

My attempt:

<a data-bind="attr:{'href:tel':telephone}">call us</a>

Answer 1

OK, I got it now:

<a data-bind="attr:{'href': 'tel:'+telephone()}">call us</a>