I don't see the benefit
Like Spike said - engineers don't work Sunday so all you can do is report a fault and wait. Same as you could do on a Monday. It might help those with anxiety disorders. It's a clever bit of smoke and mirrors. My company does the same - lines open 07:00 til 18:00 5 days a week, and 08:00 til 13:00 Saturday, but the engineering team that manages the engineers is only open 09:00 til 17:00 Mon-Fri. This is true of a few other teams as well. The customer thinks they're getting better service but they're not. Phone with a fault on Friday at 17:30 and you may as well have phoned at 09:00 on a Monday - service is no faster.