A bit of checking out stuff today.
I checked my 3.5mm 4-pin jack cable with the multimeter. All pins connecting correctly to the corresponding pins and no shorts. But - when plugging this lead into my HTC One M8 mini phone, the earphone indicator does not come on! Checked it with various other jacks (2-pin, 3-pin) lying around, and the indicator did come on, as it did with the 4-pin jack from the phone's supplied earbuds/mic headset. Not sure about this one, but will pick up another lead while I'm out today for comparison.
The lead does trigger the indication on the Samsung Galaxy Tab 3. However I'm not yet fully convinced that this tablet is "hearing" from the mike in the headset when connected, or still via the built in mic on the casing.
I've measured the voltage between pins 3 & 4 (ground and mike respectively) on both the phone and the tablet, and get 5.2V in both cases, which should be more than enough for both to work in conjunction with the Wolphi-link interface. I'm particularly intrigued that I can achieve this reading from the phone when the phone itself can't see that it has anything plugged in!
Anyway more later. Meanwhile, if these latest observations trigger any ideas in anyone - or indeed hilarity at my bumbling ineptness, then please reply with your suggestions/laughter as appropriate.
Now off to buy a Christmas card (need ONE more), see 101 year old Nanna, and buy a cable...
And yeah, I'm going SIM only when February comes around.