Let's do this again. Show me where in the contract this is at. UPS can layoff seniority drivers, preloaders, unloaders etc when there is not enough volume. You are NOT guaranteed to work just because you have seniority.
In most areas, being on call is the same as being laid off. Now here is where it gets tricky.
You are entitled to unemployment if you do not work anytime during the week. If you are on call/laid off and UPS calls you and asks you to come in and work, and you elect not to, your unemployment for the week can and will be denied.
If you elect to come in, you then have a guarantee of 8 hours of work.