If by liquid phosphate you mean a water solution of a phosphate salt at atmospheric pressure and room temperature (up to 60 ºC or 140 ºF is OK), such as sodium phosphate used to condition boiler feed water, or any other similar application, then I'd recommend three kinds of materials I've used successfully for many years in engineering projects.

1st. The tank is small, say 2 to 3 cubic meters (70 - 105 cuft): use polypropylene. Is cheaper than lined steel and performs very well.
2nd. The tank is larger, but less than 20,000 cuft.: use FRP (fiberglass reinforced polyester). It's also cheaper than lined steel and performs very well.
3rd. If for some reason you do want a metallic tank, then use a rubber lined carbon steel tank. Rubber withstands very well phosphoric acid and water soluble phosphate salts up to a temperature of 90 ºC or 160 ºF, but to have a safety margin let's say 80 ºC or 140 ºF.

Now, if "liquid phosphate 403 CT" is a special material different than the water solution of a phosphate salt or phosphoric acid, let me know it and I'll check if I know of a suitable material for it.

Regards
Giovanni S. Crisi
Sao Paulo - Brazil

Tim,

We already made the same as yours (its a transportable tank) and we used carbon steel with rubber lining. I suggest you use two layer of 1/8" rubber lining. This is cheaper than stainless or any othe exotic materials used in the chemical industry.