Here is a link to more information on the Way2Call modem.
It states there is an SDK available. It does have some fairly powerful features (that the HAL modem does not have), so I'll admit I was a little hastey in dismissing it because of it's price.
One feature it has that many HAL users wish they had is the ability to que an incoming call while it identifies the caller ID
and then put the call to voicemail, disconnect or release it to your local phones.
But over $300?? If paying $95 for the HAL modem brought a few tears to my eyes, buying the Way2Call would probably induce convulsions and the severe wrath of SWMBO!