Simplest system Protect yourself by using an inexpensive audio dongle. 
Hardware for dialing out of filemaker phone line interface
Simple audio interface
Jim Earl (jre@earldom.UUCP) has shown out very simple circuit for connecting telephone signal to SSI202 DTMF decoding chip audio input. The circuit is basic 600:600 ohm transformer isolation circuit with capacitor in series with primary circuit and potentiometer for setting the output signal level. The circuit us designed to be used in parallel with existing telephone or other telecom equipments.
.22 uf 10k pot
400v ||(----------->
Phone line tip o-----)(----)||( <---o to SSI202 input
)||( >
Phone line ring o-----------)||(-----------o---o ground
This circuit works as designed with the chip specified, because it has high impedance input. The design has one problem: If this circuit is connected to low impedance input and the output potentiometer is set to maximum level, the low impedance is reflected to primary side of the transformer, which is not good.
My recommendation is to change the capacitor size to to 0.1 uF and add one 4.7 kohm resistor in series with circuit input or output. That will keep the circuit from disturbing the telephone line when connected to other circuits.
.22 uf 10k pot
400v ||(----------->
Phone line tip o-----)(----)||( <---o to SSI202 input
)||( >
Phone line ring o---\/\/\/--)||(-----------o---o ground
4k7
https://github.com/Lord-Kamina/SwiftDefaultApps/releases
You can change it using a utility called RCDefaultApp - Alternative freeware - which installs as a Control Panel. It's old but still works right up to El Capitan [I've not tested on Sierra but see no real reason it shouldn't still work.]
Look for 'tel' in the URLs list & change...
Primary 600 ohm Secondary 600ohm+600 ohm
Tip >------------/ II /------------< audio to telephone line
/ II / (low impedance output)
/ II /
/ II /
/ II /
/ II /------600ohm--Ground
/ II /
/ II /
/ II /
/ II /
Ring >-------------/ II /-------------> audio to mixer
(high impedance input)
Fortunatly the protection on the transformer secondary is usually quite easy, because the transformer itself redices the energy which can pass through it. Typically a pair of zener diodes (voltage of few volts) connected to the transformer secondary can do the protection nicely.
-+
| +----+----------- Audio in/
| || | _|_
) || ( \
) || ( /_\ zener
) || ( |
) || ( \ /
) || ( _\_ zener
| || | |
| +----+----------- Audio ground
-+
The zener diodes will limit the signals on the transformer output to around the zener diode voltage + 0.7 V range.
In audio output circuits where the signal levels are fraction of volt range you can use normal diodes (like 1N4148) in the following way to do the protection:
-+
| +----------+----- Audio out
| || | |
) || ( Diode |
) || ( +-|>|-+
) || ( | |
) || ( +-|<|-+
) || ( | Diode
| || | |
| +----+----------- Audio ground
-+
The diodes on the secondary of the transformer connected in this way will limit the signal levels to less than 0.7 Vpp.