* Mobile phone operator cuts dividend payout
* Shares down 4 pct; Vodacom blames one-off factor
* Annual revenue up 8.1 pct at its South African operations (Recasts, adds shares, details on South African ops and smartphones sales, rival comparison)
May 14 (Reuters) - South African mobile phone operator Vodacom Group disappointed investors with lower than expected full-year earnings and a cut in its dividend payout on Monday.
The company, owned by Britain's Vodafone, blamed a one-off hit from its purchase of a stake in Kenya's Safaricom for the disappointing earnings, which missed analysts' estimates and sent its shares down nearly 4 percent in Johannesburg.
Vodacom acquired a 35 percent stake in Safaricom last year as part of a move by its British parent to consolidate two of its African interests.
On Monday it said its earnings for the year to March 31 were weighed down by shares issued to acquire the Safaricom stake, but the one-time effect had now been absorbed.
Headline earnings per share, the main profit measure in South Africa that strips out certain one-off items, was 923 cents, unchanged from a year earlier and falling short of analysts' estimate of 927 cents.
However, South Africa's biggest mobile phone operator said full-year revenue rose 6.3 percent to 86.37 billion rand ($7 billion), as it added more customers and sold more smartphones in its domestic market.
Its shares fell as much as 4.5 percent and were down 3.96 percent at 150.8 rand in late morning trade, the top loser on Johannesburg's benchmark Top-40 Index, which was flat.
Vodacom, which has invested heavily in its network to provide faster internet services as increasing numbers of consumers use smartphones, said it added 7 million customers, including 4.5 million in South Africa, in the year.
It now has more than 103 million customers, including Safaricom's 1.4 million users.
Vodacom said revenue rose 8.1 percent at its South African operations to 70 billion rand, boosted by strong device sales, with equipment sales up 15 percent.
It announced a final dividend of 425 cents per share, down from 435 cents last year. For the whole year, total payouts amounted to 815 cents per share, compared with 830 cents in the previous year.
MTN Group, Vodacom's nearest rival, cut its 2018 dividend by about 30 percent in March, but adopted a "progressive" dividend policy, saying it would use this year's figure as a base to increase payouts by 10 to 20 percent in the next three to five years. ($1 = 12.2648 rand) (Reporting By Justin George Varghese in Bengaluru; Editing by Subhranshu Sahu and Susan Fenton)