Maybe I'm overlooking something, but does the phone have to have a serial interface jack on the outside? If these phones are built on the G24 base, which I'm assuming is the same across the product line, then they should all have TTL serial IO, whether it's on the outside of the phone or not. The case could be cracked open and I'm sure you could find the lines for TX and RX.
Or are we talking about the feature being enabled on the G24 chipset, hence it would be on a jack anyway?