Ah, I missed the uk approved comment in the OP.
Also, I've never had 75 volts out of a home PSTN line. Been a long while since I've played with PSTN, but I remember it being low voltage, the max I remember when the phone was "off-hook" was approx 10v. But anyway, that wasn't the point of the thread.
Since you need it to be approved, would it not be cheaper to get a 2nd hand X110P card & use a PC to output the audio?