Instruction: Extract information from the supporting document to answer the given question.

Input:
Question: I see 2 jacks is one of the jacks Line in and the other used to plug in a telephone?
Document: I purchased this for one of my customers. I had just installed and AIO PC for him and he needed a modem to behave as a phone dialer to use in conjunction with his CRM application. I specifically chose this modem because it has the necessary 2 ports, one for the phone and one for the phone line. Most other USB modems just have the single port. It installed with no drama under Windows 7 Pro (x64).
I purchased this modem for one purpose: dial contact numbers listed in Microsoft Outlook. In case you are wondering how to do it:  Simply unplug your phone from the cable modem, insert a 2-way splitter, reattach your phone line to one input of the splitter, and  run a second line to your new external modem, attached to your computer. All done:  You can start dialing from Microsoft Outlook, or any computer program.
Used with Phonetray on win 8 system to block unwanted telephone calls. Easy to install go online to access latest win 8 driver.
I have been searching for a dial-up Modem for my Samsung laptop which has Windows 8 64-Bit O/S & the Hiro H50113 is the onlyunit I have found that works on this system.Simple to use & just plug & play setup when I'm in an area where there is no Wi-Fi but a land line to connect to the internet.Fully recommend this product to those in a similar situation.
Received Jan 18, 2014, 2 day delivery with USPS, installed with no problem on win7 64 bit HP Pavilion which has no room for internal voice/ fax modem so needs external modem,  using the disk they provided. I use on AT&T analog lines with Windows Phone Dialer voice calling program, with telephone  numbers from Filemaker pro 12 data base as well as faxing  Windows documents  when recipient does not want email. I will increase rating later if it continues to work as promised but it  tested great so far
Modem responds to normal ATI commands from a terminal window, but does NOT respond to any commands via the telephone line.The log shows no evidence that any data was received.Tried on Windows 7, Windows XP, and Vista, 3 machines.Useless piece of hardware.
Have used this modem for about 6 months, and it worked flawlessly - until I upgraded from Windows 8.0 to Windows 8.1. Contacted manufacturer, and they provided excellent instructions to modify a line in the System32 folder that made it work perfectly again! Great product, great price and GREAT support!
I tested it with a dial up internet service (NetZero) and it does indeed work perfectly in that usage. Computers nowadays lack a modem so this fills the bill perfectly. Obviously, you only need this for dial-up internet access over the telco copper pair, or alternately is can be used to allow the computer to detect caller ID info from the phone line.I bought this modem to use with a computer as an answering machine/call blocker (using CallClerk software).
The HiRO model H50113 USB modem very definitely IS voice compatible and it has caller ID. I have verified both of these features with a telephone answering application I have. It is one of the very few modems I've found for which both of these features work in 32-bit Windows 7 (I haven't tried 64-bit Windows 7).
I ordered this modem to use with an answering service software that I have. Installation is very easy, basically just plug it into a USB port, connect the phone lines. I don't have a chance to test the fax capability, but modem and voice works well so I expect the fax function will work. The trick is in the driver: skip the drivers that come with the CD disc in the package, also skip the link that was posted in one of the reviews here.
Answer: You are correct; one is for the telephone line from the wall and the other is for the telephone receiver at the desk.  In my case the receiver on the desk is the master station for seven cordless handsets in seven locations in the house, so the whole house is covered.