I have the same problem with a HandyTone 502 adapter.
Hello,I can't get call transfer to work with a SPA2102 adapter.I don't think it has something to do with FS, but I'm hoping someone here could help me.I do not get a new line in the phone (by pressing the R button), all DTMF tones are sent as audio to the other connected phone.Anyone got it working?Thanks,Jonas