Just my view as a marijuana addict myself, and not quite an answer to your question. But I wanted to share in case it helps.
When you ask, "What are your plans for the weekend?" the honest answer is probably that he is planning to smoke a bunch of dope. If he doesn't have any, his plans are probably to get some. He probably knows that this answer is not going to be one which you are pleased with, so "I don't know" lets him avoid giving you the honest answer, which you wouldn't like anyway.
As far as the cell phone, if he knew why he lost it or where it was, he wouldn't have lost it. (Either that, or if he's desperate for money, he could be selling/trading them for drugs.) You can install apps on any of the new smart phones that will let you get the GPS location of the phone if it gets lost (either from another phone, or by logging in online).
"what do you want to do with your life?" When you're a marijuana addict, what you REALLY want to do is smoke weed. Nothing else really matters. A job is just to get money so you can smoke weed. Friends are just people you smoke weed with. He knows you don't want the answer "I just want to get high all the time", so he says, "I don't know"
Honest communication with addicts isn't possible because they will lie to protect their addiction. Every single time.
Your son probably knows that your are not OK with him being a marijuana addict, so he is not going to be honest about it with you. (I'm not trying to imply that you should be ok with it or anything, just trying to explain why son isn't open and honest.)
How old is your son? Why are you concerned about what happened to his phone? (I am guessing because you paid for it. If that's the case, I simply wouldn't buy him another one if he shows he is unable to care for them.) Then you don't need to care that he can't keep track of one. If he really wants a cell phone, he could get a job and pay for his own. He probably wouldn't lose it then.