Plain old telephone service (POTS) is a retronym for voice-grade telephone service employing analog signal transmission over copper loops.[1] POTS was the standard service offering from telephone companies from 1876 until 1988[2] when the now-obsolete Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI) was introduced, followed by cellular telephone systems, and Voice over IP (VoIP). POTS remains the basic form of residential and small business service connection to the telephone network in many parts of the world. The term reflects the technology that has been available since the introduction of the public telephone system in the late 19th century, in a form mostly unchanged despite the introduction of Touch-Tone dialing, electronic telephone exchanges and fiber-optic communication into the public switched telephone network (PSTN).
Although you may still be able to use Pots service from the local exchange carrier (in the mid-atlantic it is Verizon)  many local exchange carriers are selling off the embeded base and service is beoming more of an issue with the outdated infrastructure.   There are other ways also available to simulate pots service such as a Comcast modem using Sip to analog or even a Pri which converts to analog via etensions in the PBX.  Some of the main issues with POTS are :

                1) Inability to conference more than 2 parties becuase of Volume Drop
           2) Unable to do Mobile call control (sending calls back to the host system                           on tranfer.