well a phone line is operating

We can find out how well a phone line is operating by using
Ohm's law and an ammeter. The DC resistance of any device
attached to the phone line is often quoted in telephone company
specifications as 200 Ohms; this will vary in practice from
between 150 to 1,000 Ohms. You can measure the DC resistance of
your phone with an Ohmmeter. Note this is DC resistance, not
impedance.
Using these figures you can estimate the distance between
your telephone and the telephone exchange. In the United States,
the telephone company guarantees you no lower current than 20 mA
- or what is known to your phone company as a "long loop." A
"short loop" will draw 50 to 70 mA, and an average loop, about 35
mA. Some countries will consider their maximum loop as low as 12
mA. In practice, United States telephones are usually capable of
working at currents as low as 14 mA. Some exchanges will
consider your phone in use and feed dial tone down the line with
currents as low as 8 mA, even though the telephone may not be
able to operate.
Although the telephone company has supplied plenty of nice
clean DC direct to your home, don't assume you have a free
battery for your own circuits. The telephone company wants the
DC resistance of your line to be about 10 megOhms when there's no
apparatus in use ("on hook," in telephone company jargon); you
can draw no more than 5 microamperes while the phone is in that
state. When the phone is in use, or "off hook," you can draw
current, but you will need that current to power your phone, any
current you might draw for other purposes would tend to lower the
signal level.
The phone line has an impedance composed of distributed
resistance, capacitance, and inductance. The impedance will vary
according to the length of the loop, the type of insulation of
the wire, and whether the wire is aerial cable, buried cable, or
bare parallel wires strung on telephone poles. For calculation
and specification purposes, the impedance is normally assumed to
be 600 to 900 Ohms. If the instrument attached to the phone line
should be of the wrong impedance, you would get a mismatch, or
what telephone company personnel refer to as "return loss."
(Radio Amateurs will recognize return loss as SWR.) A mismatch
on telephone lines results in echo and whistling, which the phone
company calls "singing" and owners of very cheap telephones may
have come to expect. A mismatched device can, by the way, be
matched to the phone line by placing resistors in parallel or
series with the line to bring the impedance of the device to
within the desired limits. This will cause some signal loss, of
course, but will make the device usable.