1) 2b -> asterisk IVR -> 2b out ---- Tried, very good.
2) 2b -> asterisk IVR -> PSTN out ---- Have not tried, no comment.
3) PSTN -> asterisk IVR -> 2b out ---- Tried, very good. (SZ PSTN --> VoIP --> HKBN 2b)
4) VOIP -> asterisk -> 2b out ---- Tried, very good.
I do not know the limit of outbound and inbound calls. As far as I know, if you keep the number of input bound and outbound calls within the limit of two, there should not be any problem. However, you may have more calls beyond two. You may be banned if HKBN 2b company discovers this.