We can’t apply Asterisk for VoIP if we don’t have a static IP, right?
I use www.no-ip.com to resolve my dynamic ip address, and connect to the url from my software SIP phone no problem 
Just make sure you either run the software, or have a router capable for updating your ip address with whoever you use
No, that’s not true at all. But it depends what you want to do with it. For example, i wouldn’t suggest operating as a commercial internet telephony service provider on a dynamic IP address! 