If you buy all of the items in Heracross's shop (and do a certain extra thing involving a purple guest) then they will get desperate having nothing to sell -- and they will sell you some silly things! The phone is one of them.
thank you! I had used both, but grimer comes so infrequently I just didn’t know if anything had changed or not, so I’ve bought the happy meal toys and now I just need to find out how to get the cell phone and whatever else there might be in secret
Yay! Yeah I can't remember if the "grimer happy meal toys" arc needs to completely end for that other stuff to show up, but you should be on your way now.