Mr Coulson emailed the instruction to his then head of news, Ian Edmondson, prosecutor Andrew Edis QC said.
They feared a rival paper may get the story about the son of George Best and a woman, Mr Edis told the Old Bailey.
Mr Coulson and Mr Edmondson deny conspiracy to intercept communications.
Mr Edis said that in 2006, the now-closed News of the World
(NoW) was investigating Calum Best, who was thought to be the father of a
child with a woman who was willing to sell the story.
The NoW wanted the story as an exclusive and were paying the
woman a lot of money, but were worried that Mr Best - whose father was
the late Manchester United footballer George Best - might "leak" the
story to their competition, the court heard.
Following an email discussion on the matter, Mr Coulson sent Mr Edmondson a message which read: "Do his phone."
"What does that mean?" Mr Edis asked the jury.
He added the evidence against Mr Edmondson was "overwhelming"
South African Anene Booysen rapist jailed for life
A South African man found
guilty of the rape and brutal murder of a 17-year-old girl has been
sentenced to two life terms in prison by a court near Cape Town.
State prosecutors had requested this sentence - without parole - for 21-year-old Johannes Kana.
Anene Booysen died in February, hours after she was mutilated in a case that caused national outrage.
South Africa has one of the highest rates of sexual violence in the world.
'Shocking and inhumane'
The BBC's Nomsa Maseko at the court in Swellendam, 220km (136
miles) from Cape Town, says Kana's uncle clapped his hands and laughed
as the judge announced the sentence.
"This is a joke," he said, adding that the system was unjust.
Many of those attending the trial feel justice has been served, our correspondent says.
Germany allows 'indeterminate' gender at birth
Germany has become
Europe's first country to allow babies with characteristics of both
sexes to be registered as neither male nor female.
Parents are now allowed to leave the gender blank on birth
certificates, in effect creating a new category of "indeterminate sex".The move is aimed at removing pressure on parents to make quick decisions on sex assignment surgery for newborns.
However, some campaigners say the new law does not go far enough.
As many as one in 2,000 people have characteristics of both sexes.
Germany hopes for Snowden meeting on US spying
The German government
says it is keen to hear directly from the fugitive NSA whistleblower
Edward Snowden about the US spy agency's activities.
"If the message is that Mr Snowden wants to give us
information then we'll gladly accept that," said German Interior
Minister Hans-Peter Friedrich.
Mr Snowden's lawyer said a meeting could occur in Moscow, but not Germany.
The US Secretary of State, John Kerry, has said that in some cases, US spying has gone too far.
He said he would work with President Barack Obama to prevent further inappropriate actions by the National Security Agency.
Mr Snowden, 30, fled to Russia in June after leaking details
of far-reaching US telephone espionage. He has temporary asylum,
allowing him to live in Russia until next June.
In a surprise move, a German Green MP, Hans-Christian
Stroebele, has met Mr Snowden in Moscow and revealed the former
intelligence contractor's readiness to brief the German government on
NSA spying.
Mr Snowden set out his position in a letter, which Mr Stroebele showed to reporters at a news conference on Friday.
The scale of the alleged US espionage has provoked international concern and calls for tighter supervision.
Asian countries have protested at claims that Australia was involved in a US-led spy network.