Well there are two requirements in order for a modem to be compatible with IVM:
1) It must be TAPI compliant (the modem has to be built this way by the manufacturer, it's not something you can just add on).
2) It must be Voice capable (Usually labelled as "Voice Modem")
If the modem is not TAPI compliant, it means IVM has no way of communicating with it and instructing it to perform functions like answering calls.
If the modem is not Voice capable, IVM cannot send voice/sounds through the modem to the other end, nor can it receive the voice/sounds, so it is useless. Regular modems such as the one you purchased are designed only for dial-up internet and faxing applications.
TAPI compliant Voice Modems typically cost a little more than regular data/fax modems, but are still very affordable (around $35).