As long as you get the call-in with a good buffer before VPs count down, the CP cost doesn't matter. There isn't really anything in the game that truly punishes camping right now with how inefficient any non-doctrinal artillery is, and even doctrinal artillery is fairly useless.
Call-ins need a far higher resource cost. But if you add the resource cost into the call-in cost then they're never ever worth it when teching, only when skipping tech. So really, they should just require tech. It's the simplest solution. Also I don't agree that specific call-ins should require specific tiers for soviets.
Couldn't agree more
T3 or T4 for any soviet tank call-in, and perhaps battle phase 2 for command tank/puma and battle phase 3 for tiger/elephants.