"Stuart"
However, regardless of what you know or don't know, this thread was
started by someone who wanted to know whether he could re-use the
transformers in telephones for another purpose so clearly he has some!
** That is entirely false.
What the OP *actually asked* was this:
" If I strip down some landline phones I 've got here, then will there
be a matching transformer in each one? "
Chances are high there are no iron core transformers ( hybrid or other) in
his phones at all - cos they are too modern.
Secondly, his purpose requires a transformer with high voltage safety
isolation from the phone line - which the common 600:600 ohms phone line
types all have and others do not.
Thirdly, his purpose requires a transformer with very high CMRR - cos that
is why he is getting humming noises at the moment when making recordings
with his passive ( ie non-transformer ) adaptor device.
Have you ever done this ??
I can assure you it ain't as simple as you suspect to get a hum free and
distortion free result.
Not the least of the problems is that portable voice recorders have high
sensitivity mic inputs with automatic gain control circuits - which cannot
be defeated by the user. This results in annoying gain pumping and regular
bursts of overload distortion PLUS increasing background noise & hum
whenever there is a pause in the conversation.
The trick is to use just the right amount of resistive divider attenuation
before and after the 1:1 transformer to eliminate this - plus cap couple
the input side to stop pulling the line low.
..... Phil