Cesium nitrite
The compound CsNO2 is called cesium nitrite. It is composed of cesium (Cs) cation and nitrite (NO2) anion.
The name phosphite is according to IUPAC reserved for PO33- ion, so Cesium phosphite would be Cs3PO3 However no compounds containing PO33- are known to exist. Hypothetical compounds containing it are often used in naming exercises by teachers/colleges/web sites. Phosphite in common use refers to HPO32- ion (IUPAC name phosphonate) and this is the formula taught in some parts of the world, cesium phosphite would therefore be Cs2HPO3