I found this on the state of Istanbul:
"Meanwhile, the siege of Istanbul would take far longer, and by the end of it, much of the city would be in ruins, even seriously damaging the Hagia Sophia."
This indicates the siege has been ongoing as I don't think it has ever been mentioned as being broken or the city taken. After all being forced to withdraw would be a major blow to Russian morale and a great victory for the Allies.
"Meanwhile, the siege of Istanbul would take far longer, and by the end of it, much of the city would be in ruins, even seriously damaging the Hagia Sophia."
This indicates the siege has been ongoing as I don't think it has ever been mentioned as being broken or the city taken. After all being forced to withdraw would be a major blow to Russian morale and a great victory for the Allies.