Istanbul (AFP) - When making a social phone call, you don't expect to hear the voice of the head of state.

But mobile phone users in Turkey were surprised to hear a voice message by President Recep Tayyip Erdogan when placing a call around midnight on the anniversary of the failed July 15 coup.

After dialling a number, instead of hearing a dialtone users heard a voice message from Erdogan congratulating them on the national holiday of "democracy and unity" that marks the coup's defeat.

Only after Erdogan's message did the dialtone begin.

"As president, I send congratulations on the July 15 National Day of Democracy and Unity and wish the martyrs mercy and the heroes (of the defeat of the coup) health and wellbeing," said the message read by Erdogan.

The message could be heard on Turkey's largest network Turkcell, AFP correspondents said. The Hurriyet daily said it was also heard on the Vodafone network.

"People who wanted to chat on the telephone got an Erdogan surprise..." said Hurriyet.

Turkcell had already sent text messages to clients promising them a free extra gigabyte of data from July 15 in celebration of the coup's defeat.

Users had in the wake of the coup bid received text messages from Erdogan. But this is believed to be the first time a voice message has been employed.

"Enough now... you even get it on the phone... how can there be such an insult, it's like a nightmare," opposition lawmaker Aykut Erdogdu wrote on Twitter.

Also just after midnight salah prayers for victims of the coup bid rang out from minarets of 90,000 mosques across Turkey, state media said.