Session initiated protocols. I know what it stands for; but don’t really know what it means. I just read a lot about VOIP phones… Well I don’t know anybody who uses VoIP phones.
Does SIP providers connect two normal phones with no apps, addons, plugins. Just regular normal phone calls to my regular normal number given to me by Verizon?!
Say I want a server to connect two people; they both have stock iPhone’s one on AT&T and one on Verizon. Is this possible?! 
SIP doesn’t stand for session initiated protocol!
The term SIP Provider is normally used to refer to business that is connected to the PSTN on one side and SIP on the other and therefore allows one to terminate calls over SIP that arrived over the PSTN and to originate calls over SIP that terminate on the PSTN. As Asterisk is a back to back user agent, it can connect the two types of call together.
Note that Asterisk not exclusively a SIP PABX and was originally designed for direct PSTN connection and still supports that with the right hardware.