I'm not familiar with TAPI, but from a quick glance at the Microsoft documentation, it appears to be a COM interface. So, there's probably no reason you couldn't use it from LotusScript, starting with a call to CreateObject, much as you would in Visual Basic. The main difference is that you have to declare all your object variables as Variant instead of their specific types, since LotusScript doesn't support forward binding for COM.
http://lotuspro.e-promag.com/garyspage contains several presentations and examples of the use of COM/OLE2 from LotusScript. The Domino Designer help also has information about calling COM APIs.
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This was first published in June 2004