The High Court in Johannesburg will rule on Thursday on an application by Cell C for an urgent interim interdict against a banner.
The banner was put up by one George Prokas at the WorldWear Mall on Beyers Naude Drive on November 6.
It bears a logo resembling the Cell C one and read: "The most useless service provider in SA -- Cell C Sandton City."
It gave the name of the franchise manager and his phone number, claiming he had said that his "unnamed executive head refuses to assist the customer".
Christopher Whittcutt, for Cell C, argued on Wednesday that the banner was a defamatory statement.
Shem Symon, for Prokas, questioned how the cellphone provider could ask for an interim interdict for something that had already happened.
"I don't really understand, after it has already happened and after it has gone viral," he said.
The dispute arose from a phone Prokas had taken up for his son with Cell C in 2013. The phone's indicated number turned out to belong to and was being used by a different person, who ran up a bill of R5000.
When attempting to buy a car earlier in 2014, Prokas discovered he was listed as a bad debtor as a result of the unpaid R5000.
He sought to have the bill waived by Cell C but that did not happen. Prokas exchanged e-mails with Cell C, indicating he would put the banner up unless his complaints were dealt with.