You have no responsibility to be "able" to do an IA, and if you can't then you can't.
First, you don't have to answer the phone, OR CALL BACK if they leave a message.
Second, if you do answer the phone, you can absolutely say, "sorry, I'm sitting at a stream in the woods where I was dropped off, camping, with no car. Can't make it." or hiking in the wilderness 2 days out, etc.
Third, you can absolutely say, "Dude, it's a day off. I've been drinking beer all morning watching football!"
Fourth, you can be watching the kids with no child support replacement.


From the current "When Scheduling Calls":

c. You may not be compelled to:
5) Accept an IA as a regular line holder for a rotation with a
report within 11 hours of your release at your base.
(Section 23 R. 7.)

and:
A reserve pilot:
• On long or short call must be contactable as described in Section 23
S. (See “Reserve” on page 27.)
• On a layover has no obligation to be contactable.
• On an X-day has no obligation to be contactable or to be in a position
to report for an inverse assignment.
A regular pilot:
• Has no obligation to be contactable at any time that he is not actually
on a duty period (i.e., between report and release on each duty
period).
• On a day off has no obligation to be contactable, or to be in a
position to report for an inverse assignment.

edit-- doh, I researched too long, several guys already answered this...