[TMSZ]

V.23 FSK is just the name of modulation. You can have CLIP receiver as separate IC (https://www.microsemi.com/product-directory/caller-id/4305-m...) or as some DFT code with Goertzel algorithm with maybe 0.1 MIPS DSP budget allowed. No sane person would add full modem capabilities to this.

That said, PABXs I worked with have built-in software modems (both POTS and ISDN, needs to be explicitely enabled) with remote management capability and there is also dedicated web portal for management even if device is behind NAT (paid feature). Whether you want to trust hardware/software you have no control of - that's another story. For "big" PABXs partnership between manufacturer and installers usually lasts for years.