I think you are saying you have a POTS/PSTN line currently installed at your location that you want to connect the SPA 112. The SPA does not perform that function. The SPA is an ATA (Analog Telephone Adapter) used to connect non VOIP devices to VOIP networks. To connect your phone system to the local lines a "Gateway" is the connection which allows your VOIP system to connect to the local POTS/PSTN lines. But if you have a VOIP service provider they can do that as well. I would expect you want the POTS/PSTN local lines for a backup in the event of internet connectivity outages/issues.
To piggyback on what Jim said. If you are trying to send some calls over VoIP and some over PTSN, then what you would need is indeed a gateway. You also a need a routing plan that will fit your purposes. If you want certain numbers to go out to the PSTN and some to go out via VoIP then that is accomplished by making routing patterns, e.g. if you want every dialed number that starts with 9 to go out to the PSTN, then you would possibly use the pattern 9*.
If you are planning to use PSTN as a backup in the event that your VoIP connection fails, then that would be a different implementation.
Here's a link to an explanation on call routing, it's for CUCM 8.6, but the same basic principles apply to most call gateways: http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/admin/8_6_1/ccmsys/accm-861-cm/a03rp.html#pgfId-1056007
Article ID:1966 Network Address Translation (NAT) Settings for Lines on
PAP2T and SPA3102 Phone Adapters Objective The objective of this
document is to configure Network Address Translation (NAT) settings for
the lines of communication on the PAP2T and SP...
Article ID:2742 Speed Dial Settings on SPA100 Series Objective Speed
Dial Buttons are often used to make the process of calling regularly
used contacts quicker and with one button. This option is used in place
of dialing the entire phone number in. This h...
Article ID:2676 Firmware Upgrade on SPA112 and SPA122 Objective To
upgrade the firmware a specific file must be downloaded from the
Internet and uploaded to the device. Upgrading your devices can be very
useful. Firmware upgrades can fix software bugs, im...