Map a new route with a constraint:

routes.MapRoute(
    name: "",
    url: "{tel}",
    defaults: new { controller = "sales", action = "quotes" },
    constraints: new {tel = @"\d{9}"}
);

This will map any URL that consists of exactly 9 numbers (and nothing else) e.g. mydomain.com/123456789 to the "quotes" action on the "sales" controller. Register this before the default route. It won't affect existing routing.

https://docs.microsoft.com/en-us/aspnet/mvc/overview/older-versions-1/controllers-and-routing/creating-a-route-constraint-cs

You will want to confirm the expected format of the telephone numbers with the customer and adjust the Regex expression in the constraint accordingly.