MTN Group Ltd. has been ordered by a Johannesburg court to allow customers to keep their phone numbers when leaving for South African telecommunications competitors.

The policy of Africa’s biggest wireless operator by sales to block subscribers from retaining numbers if they did not apply to transfer them within a certain time frame was challenged in the city’s High Court by Cell C Pty Ltd., South Africa’s third-biggest mobile-phone company. The court ruled in Cell C’s favor on Tuesday, according to legal documents.

“MTN takes note of the judgement and can confirm that we will comply with the contents,” Graham de Vries, head of corporate services at Johannesburg-based MTN, said in e-mailed comments. The company will refer the matter to the Independent Communications Authority of South Africa, the industry regulator, he said.

MTN shares declined 1.8 percent to 120.35 rand as of 4:04 p.m. in Johannesburg. The stock is down 9.4 percent this year, compared with a 1.9 percent gain at crosstown rival Vodacom Group Ltd.

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