Phosphates play a role in different foaming processes. In slag foaming, the phosphate can lower the surface tension and increase the foam life time. In the sintering–foaming process, sodium phosphates are added as “foam stabilizers”. Though, phosphates show documented effect in slag foaming, the effect of phosphate in the sintering–foaming process still needs to be clarified. In this work, we investigate the influence of alkali phosphates (Li3PO4, Na3PO4, and K3PO4) on the glass foaming process, foam density, and glass transition temperature (Tg) of waste silicate glasses. The results show that the Tg of foam glasses decreases with increasing the alkali phosphate content, and with decreasing the alkali ion radius. The results, therefore, indicate that alkali phosphates can be used to decrease the heat-treatment temperature, however, only K3PO4 and Na3PO4 prove to be promising candidates to obtain low density foam glasses.