The first mention of USS Callao was on Otis' Further Instructions to Miller on December 26, 1898, just like the steamship S.S. Union.
Manila, P. I., December 26, 1898.

Brig. Gen. M. P. MILLER, U.S.V.,
Commanding First Separate Brigade, Eighth Army Corps.

SIR: I am directed by the major-general commanding to furnish you with the following instructions:

In case the Spanish forces have evacuated Iloilo, you will, on approaching that city, keep your transports Arizona and Pennsylvania well to the rear and beyond the view of the inhabitants of Iloilo, and you will take into the Iloilo waters your naval escorts, the Baltimore and Callao, and the transport vessel Union, on which are the native Spanish troops, with your vessel, the Newport. You will make known to the insurgent authorities the object of bringing the large force with you, which is, viz: To take possession of other ports in the islands, if conditions and circumstances are favorable, but that it maybe necessary to keep troops at Iloilo until definite instructions concerning other ports of the islands are received from Washington.

Very respectfully,

Assistant Adjutant-General.

But apparently, the sending of USS Callao to Iloilo did not push through.

And upon arriving in Iloilo, Miller reiterated his need for the Callao or a low-draft boat in his first report to Otis.
Iloilo Harbor, P. I., December 28, 1898 — 3 p. m.

Department of the Pacific and Eighth Army Corps.

Sir: I have the honor to report that my command arrived in the harbor of Iloilo about 10 a. m. to-day. The Arizona and Pennsylvania were left at Point Luzaron, 30 miles away. The Baltimore and Newport anchored off the city.

An aid of the commanding general, Martin Delgado, immediately reported aboard my ship. I gave him an interview. He reported that the commanding general desired to know "if we had anything against them - were we going to interfere with them?" I informed him that i had written a letter stating to his commanding officer and the people of Iloilo the object of my visit, and would send the letter over. accordingly, Lieut. M. K. Barroll, Third Artillery, and two volunteer aids and the commission went to visit the commanding general. They were met by a subcommittee, of the committee of which R. Lopez was president, General Delgado being present. My aids gave them my letter (a copy inclosed). They wanted to know of Lieutenant Barroll almost at the very first whether he had any instructions for them from Aguinaldo. He answered no: but that the instructions were from Major-General otis, commanding the Philippine Islands United States forces. After reading the letter they claimed that they had no power to act in cases affecting their federal government, but promised to meet me on my ship to-morrow afternoon.

When we entered their flag was flying from two places in the city. At 3 p. m. today it was not flying. I presume this was because my letter claimed the authority of the Spanish Government over Iloilo, as it was abandoned by the Spanish troops.

They were polite, but i think them determined not to give us control, except "we use force, when they will yield without much fighting. They have taken charge of the custom-house and post-office. They know that our troop ships are off Point Luzaron, therefore i ordered them in to-night—not nearer than 6 miles.

The city is quiet, but the white citizens, especially americans, are afraid. Their force is estimated at 800 well-armed men, 1,000 badly armed men, and 1,000 men with guns, pikes, etc.; ammunition not supposed to be abundant.

I am told now that the members of the commission are afraid to express an opinion in our favor. The fact that their people are in possession of the city has changed the views of the many wavering ones. The longer they remain in possession collecting customs, running post-offices, the more they will be confirmed in the idea that they can do it. I should recommend that force be used at once, in which case i desire the Callao, or some other light-draft boat, and the california heavy artillery battalion sent down till the place is taken. With the forces now here and that in addition i would not expect to fire a single shot, as the native troops would move out. i will keep you informed.

very respectfully,

Brigadier-General, U. S. V., Commanding First Separate Brigade.

Unbeknownst to Miller, on that very same day, December 28, 1898, Otis received a dispatch from Dewey that it is not practicable to send USS Callao to Iloilo.
Manila, P. I., December 28,1898.

Lieut. Col. C. L. Potter,
Chief Engineer Officer, Department Pacific and Eighth Army Corps.

Sir: since delivering to you instructions for General Miller, I am in receipt of a dispatch from Admiral Dewey saying it is not practicable to send the Callao to Iloilo, and he further thinks the proper thing now to do is to recall the expedition, as the insurgents are in full possession and will probably not give up without a fight. This expression of view on the part of the Admiral only confirms my view that you should use every possible means of conciliation, and still I am not of the belief that the expedition can be returned. Better that we leave the war vessel and a small force to confront Iloilo and scatter the force to other ports in the southern islands, where troops are very much needed at the present time. You will therefore inform General Miller to be governed by these views as nearly as possible. I will try and send further information in regard to the condition of the islands tomorrow or next day. Notwithstanding all this, i still hold to my view that Iloilo must be taken.

very respectfully,

Major-General, U.S.V., Commanding.

Finally, another light draft ship was sent, the USS Petrel, which left Manila January 4 1899.

The departure of USS Petrel from Manila Harbor between 8am and noon on January 4, 1899 was observed by another ship, the USS Concord.

USS Petrel arrived in Iloilo on January 5 1899.

Miller saw the advantage to be gained in having a light draft boat for the purpose of going up the river which would make shelling possible even in the very heart of the city, destroying earthworks and other fortifications.
-Markey, 51st Iowa

But the Filipinos were aware of the purpose of USS Petrel, that upon her arrival, they sank three lighters at the entrance of the river, thus partially obstructing the channel, allowing only small steamers to pass, and blocking and preventing the USS Petrel from going upriver.

The American plan was, for the USS Petrel to go up river, followed by the small gunboat USS Samar.

Because of the obstruction, only USS Samar was able to go upriver on February 11 1899 when the Americans landed.

However, the USS Petrel was able to use its light draft feature by positioning itself close to Fort Iloilo or Fort San Pedro the day before, on February 10 1899, in which position it was able to observe activity on the shore the following day, February 11 1899, and begin bombardment.