You are being charged for airtime, which is the same regardless of who initiated the call.
It is illegal in the US for telemarketers to call a cell phone (for that very purpose). If it's a business you have a relationship with (a company you've ordered from before), that's different - but you can request to be removed from their call list and they will be back to the same category.
You are not charged for calls you do not answer, and most cell providers do not charge you to call your voicemail.
Some providers do have free incoming calls, but they make up for it in other ways - by giving you less minutes, charging more for roaming, etc.
It's not so much a monopoly, but an uncompetitive competition. Don't get me started on text messages. It costs more per-character to send a text message than it does to send data to the SPACE STATION. The cell providers have no guarantee on delivery for texts, and they only send them during idle times, so it's effectively free for the provider, 100% profit.