Not clear for what application you intend to use this modem - dialout? Dialin? Both? Character mode? PPP? Both?
Here's some links that may be useful to you:
Configuring [PPP] Dialout using a Modem on the AUX Port
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Configuring PPP Dialin with External Modems
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Modem-Router Connection Guide
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As far as using the Sportster - historically it's been one of the best consumer-grade external analog modems around, but it seems that recently I've seen fairly frequent cases where, in dialin applications, it tends to forget what DTE speed it wants to use, rendering it marginally useful for that case. (If you only need it for low speed dialin, I've found that it works to lock its DCE rate to 9600 bps then configure its DTE speed to follow DCE speed.)
Good luck,
Aaron