To the best of my knowledge in January 1863 the Pine Bluff Telegraph line ran at least to South Bend (Jourdan Plantation) at the crossing on the south side of the Arkansas River from Arkansas Post. It may have ran as far as Red Fork south of Jourdan's Plantation, because the Confederates maintained a cavalry scout picket line along the lower Arkansas River down to Napolean to keep informed as to the movement of Federal transports on the Mississippi River. I don't believe that the telegraph line actually crossed the River to go to Arkansas Post directly because of the lack of water proof wire in Arkansas. Usually what little water proof wire in the South that was available was used for command detonating of underwater torpedos. And the Taliban think that they were the first to use I.E.D.'s

It was at the Jourdan Plantation that Walkers Texas infantry Division stopped in their march to relieve the garrison at Arkansas Post during the battle. The road at this point split to cross the Arkansas River as it continued down the south side of the River to Napolean, Ark. Most historians question why Walker stopped here. Some ever think it was a lack of courage in him not to going on to relieve Arkansas Post. But had he left this critical junction unguarded by either going on to Arkansas Post or even proceeding farther south on the Napolean Road to harrass the unprotected federal troop transports from the south banks of the Arkansas, he could have easily been cut off had the Gunboats steamed past him and captured this road junction.

But to answer your question directly it would seem that communication between Arkansas Post and Little Rock appears to have been by mounted courier from the Post to Jourdan's Plantation where it was then sent by wire to Pine Bluff and then relayed to Little Rock.