Asterisk provides mechanisms to read DTMF and control dialplan execution, not IVRs; do you actually need support for FreePBX, which layers IVRs on top of Asterisk?
Asterisk can forward the customer’s number; in fact if you use Read and WaitExten, that is the default behaviour. However most service providers will either reject the number or flag it as “unscreened”, to protect callee’s against forged numbers. Some of those will allow you to prove that the number belongs to you, but that is not going to be relevant in this case.
Some additional complications arise if using SIP, because of the need to identify that the call should be billed to you, but it sounds as though you are using ISDN, os your identity is established by the physical line.