1.Aragorn would not have been looking for them with the palantir, that would be
waaaaay too dangerous. First, because it is exhausting, and second, it
would tip off S that something was up in addition to the march to the Morranon.
2. As someone else pointed out, at the exact time of the call, Aragorn and
Gandalf knew that Frodo and Sam had reached Mordor, escaped from captivity, and
were at large. While they didn't know how close they were, they knew that
the hobbits could walk about ten miles a day, and that it was about 40 miles
from Cirith Ungol to Mount Doom, and that 5 days had passed. This puts them
pretty close to the finish
line.
Some who wander are lost.
