Steven Ngolo was serving a sentence for cannabis dealing at Thameside Prison when Olamide Fasina, 25, was stabbed in October 2014, the Old Bailey heard.
Ngolo exploited a "flaw" in the jail's system to communicate with gang members on a phone meant for family calls.
He was jailed for three years for conspiracy to cause actual bodily harm. Two other men were also jailed.
Louis Henry, 22, of Greenwich, who was cleared of murder but convicted of manslaughter, was jailed for 12 years.
Alvin Ansah-Baaphy, 23, of Old Dover Road, south-east London, was jailed for three years for conspiracy to cause actual bodily harm.
Mr Fasina, who was also known as Trigger, was stabbed in his chest, stomach and arm during the attack in a street in Thamesmead on 14 October.  The attack was in retribution for a robbery on a low level drugs runner.
The prosecution said the Serco-run category B prison was "really quite hopeless" as staff did not check whether one of Ngolo's co-defendants was his cousin or brother as he had falsely claimed, the court heard.
The jury heard phone conversations recorded by the prison in which Ngolo encouraged an attack on Mr Fasina.
A drug dealer has been jailed for encouraging a fatal revenge attack from a phone in his prison cell.