Mobile revenue growth has declined in sub- Saharan Africa since 2013 and is expected to continue its downward trend until the end of the decade—despite a fast-growing subscriber base.

Much of the drop has been attributed to the use of over-the-top (OTT) messaging services like WhatsApp and Facebook Messenger. With more subscribers showing a preference to chat and and make voice calls via these platforms, there’s an “increasing cannibalization of traditional voice and messaging revenues,” according to a new Mobile Economy report by the GSM Association (GSMA) trade organization.

At stake for Africa’s telcos is the significant capital investment made to build out mobile networks. The industry is expected to spend around $31 billion to expand across sub-Saharan Africa over the next four years, says GSMA.

Telco executives argue that to see a return on that investment, voice and SMS revenue growth will need to match or outperform previous years. “You can’t adjust your costs for building the same quality of network,” a senior executive at one of Nigeria’s main telcos told Quartz, referring to a problem of “voice transference” as more Nigerians use WhatsApp in particular.

Services like WhatsApp and Facebook Messenger have been subject of debates by local mobile operators and regulators. Last year, Nigeria’s telecoms regulator claimed that amid diminishing revenues, OTT services overwhelm local operators’ networks and leave them with little incentive to invest and improve broadband capability. South African operators also have complained about OTT services freeloading on their networks.

“They don’t pay taxes, don’t develop infrastructure, they don’t even open offices and create jobs. They are undermining our industry.” complained a senior executive at one of South Africa’s largest mobile operators, who asked not to be identified as he did not have permission to speak publicly on the topic.

Local regulators keen to be seen as supportive of digital platforms favored by young people also do not want to discourage investors—or harm tax revenue—by ignoring the complaints of the phone companies.

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