Turned away - Lack of customer service

I was given the Groupon special as a gift.we were unable to place an online booking through Groupon,i tried to call the restaurant beforehand and both lines were down.the purpose of Groupon is to get customers to your business who are unaware that it existed or who have never been before,with the hope that they would be blown away by your services and return in the future,this will not be the case with us.We approached one of your waiters at 7:30pm,he said we need to have booked online and they can't accommodate us.He acknowledged there are problems with the phones but if we came in before 7pm,they would have made an exception.The restaurant was not full,what difference does it make that we arrived half an hour later?a time frame wasn't stated on the voucher.if YOUR phone lines are down,how is that OUR problem?a lady who looked like the head chef,came out and ignored myself and my fiance and asked the waiter what the problem was,not the customers.i got the same response,they can't accommodate us because they haven't prepared a meal for us and that is just how the procedure goes,we must come back.the waiter and the chef were rude and turned us away