Capt. Bunker's men were not unrolling a telegraph line to Ft. DeRussy. They were much too busy trying to keep their scalps intact to be engaging in such an activity. It would have been pointless to do so, as the Confederates were totally in control of the area around the fort, and would have either tapped into the line to listen in, or taken it down as fast as it was put up. General Banks could not continuously communicate with Ft. DeRussy - he did not even have any troops permanently at the fort at this point in the campaign. The Union control of the fort consisted of gunboats shelling the fort whenever they saw Confederates in the area, and the boats were sniped at constantly in the last few days of the campaign. Banks' only communications with the gunboats at the fort were, as you suggest, via courier. And those communications were only very occasional. At the end of the Red River Campaign, the Yankees in Alexandria were starving and short on all sorts of equipment and supplies.

And as a perhaps interesting side note, Joe Laprairie is the only Yankee soldier buried in the Fort DeRussy graveyard. All the others buried there were removed to the National Cemetery in Pineville after the war. Joe lived in the area, and was buried there in the 1930s. Until recently, some of his descendants believed he was a Confederate soldier, and were very surprised (disappointed?) to find out otherwise.