In no uncertain terms do you want a view accessing your web.config file.
If this is something which shows up in many places in your views and requires a static set variable, then make a partial view to display the number, and call the partial view multiple times.
The advantage is that you are keeping it in a re-usable area so that if you change it once, it changes everywhere, and also that you are not going to have any separation of concern violations by having your view make calls where it shouldn't have access. Updating the partial will also not require a re-compile.
/Shared/_PhoneNumberPartial.cshtml
<span>555-1234</span>
Used in a view
<div>Phone Number<br/>
@Html.Partial("_PhoneNumberPartial")
</div>