Yest it is better to use TBST for western blots if you're looking at phosphorylated proteins. Depending on your antibody, the phosphate in PBSt could create problems, so it is always better to avoid it. Actually why not use TBST for all your blots? That way you don't have to make two buffers.
I would recommend the procedure here:
http://www.mapkinases.eu/protocols/western-blot/I am quite sure you will get your results, if you follow that!
Good luck!