In fairness to the OP, Marriott is making a big deal out of use the app to request things (such as robe), etc. They've done press release about it, written up in articles, they're paving the way, millennials like apps, yada, yada.

So if Marriott is going to tout the in-app the least they could do is actually respond to those putting in the requests! Having said that, I don't think the OP needs to feel awkward about calling 'at your service'. Heck, if I were the OP I would even mention the app fail when I called 'at your service' to place the request.

Cheers.