Vodacom was hit by a slowdown in quarterly revenue growth, as its home market performed badly.
The Johannesburg Stock Exchange's All-share index was down 0.7 percent at 0755 GMT.
The rand - which has enjoyed a strong start to 2019 - traded at 13.8175 versus the dollar, around 0.05 percent weaker than its previous close.
It has tracked global markets this week, rallying on Wednesday as top trading partner China pledged more fiscal spending to support its own slowing economy.
A survey showing South African consumer confidence remained stable in the fourth quarter of 2018 did little to change expectations about the pace of an economic recovery.
Government bonds were slightly firmer in early deals, as the yield on the benchmark 2026 instrument dropped 4.5 basis points to 8.805 percent.
(Reporting by Alexander Winning; Editing by Mark Potter)
