Whenever I used to talk to dealers (thankfully I won't be needing to do that for a long time now), if it was a first time, I'd introduce myself, then tell them who I got their number from. Then, for both new and regular dealers, I'd ask if they were busy, if they said no, I'd ask if I "could swing by for a second and pick something up." Just don't say weed or any slang for it. If you keep it very vague and general when you call, the dealer will know what you want, but on the (very) off chance that his phone is bugged, it will be much harder for the cops to pin anything on anyone.