Regex.Replace(phoneNumber, @"[^0-9]", "")
There's not really a "before it's saved to the DB" place to tie into, at least not easily. I've seen other frameworks such as RoR or Django that do let you do something like define a "pre_save" method or similar that provides a hook, but Entity Framework has no similar functionality.
However, you can do it in other ways. Likely the easiest is to define a custom setter on the property:
private string phoneNumber;
public string PhoneNumber
{
get { return phoneNumber; }
set { phoneNumber = !string.IsNullOrWhiteSpace(value) ? Regex.Replace(value, @"[^0-9]", "") : null; }
}
Then, every time some value is set for that property, it will automatically remove any non-numeric characters. Alternatively, you can also just clean it up in the action before saving:
model.PhoneNumber = Regex.Replace(model.PhoneNumber, @"[^0-9]", "");
db.Entry(model).State = EntityState.Modified;
db.SaveChanges();