If you’re planning to do this in your home then you’re most likely looking for a basic analog to SIP gateway (either a card or an external appliance). If you’re in an office then you need to know what type of PBX you’re connecting too. Different PBX flavors have different connection options.
Assuming you’re looking for a home install that is drop-dead simple (and price isn’t a key concern) then I would suggest a Dialogic DMG1000. It is relatively easy to configure, well documented, and there is plenty of community support for it.
There are a number of less expensive ways to talk to your speech platform but they require a bit more research. Not all of those TDM cards convert PSTN to SIP; most use their own protocols and you need special drivers to get Speech Server to work with them. There are also 3rd party SIP gateways like Paraxip which can add SIP support to Dialogic and Sangoma TDM cards.
What sort of environment are you looking to deploy in? I might be able to give better answers with some additional information.