Cell C's Weekender promotion that allows free calls between its customers has attracted so much new business that it has strained the cellphone network's infrastructure, chief executive Jeffrey Hedberg said.
Revenue grew 18percent over the six months to end June, but it is not clear if Cell C has managed to turn to profit.
"Most costs are paid in dollars and euros, and with the cyclical nature of the rand, we don't have the net profit available," Hedberg commented.
Cell C currently has 3,4million subscribers, almost a third more than last year.
"We had phenomenal growth in turnover and our subscriber base over the past six months, especially in the past few weeks of our special promotional Weekender programmes," he said.
"But this came with congestion in some networks," Hedberg added.