Ask the person who specified the use of PHPAGI. Generally, the use of something like this should not be a given for a project unless it has been evaluated by someone who has a reasonable understanding of how to use it - typically one would prototype in an unfamiliar language before committing the design to the use of that language.
I know this doesn’t always happen with high profile commercial products, but the PHPAGI developers don’t have advertising resources to convince non-technical board members that there company will go bust if they don’t use it.
Asterisk can do IVR completely within the dialplan for simple cases, so the specification of PHPAGI should have been done by someone who can:
a) justify why Asterisk can’t do it;
b) understands how it can be done using PHPAGI
(It’s not uncommon for over-constrained projects to be home work exercises.)