You do not need optical coupling , if things what your mcu is monitoring on do not require that .
That means you can supply you mcu directly from phone line , and if it is low power mcu , you can get power even when no call is ongoing (but dc-dc converter will be needed ). I do not remember exact current (in theory POTS line is current source) may be in range 15-25 ma . So , if you get 12 V at that current - you have 200 mW of power .
Simplest way is to get power
via serialy zener and resistor . Send and receive can be done directly from line via decoupling capacitor , connected to line where resistor is present . That is for half duplex connection . But if you need something more sophisticated and/or full duplex
mode - hybrid transformer and other wires must be added (transistor or transformer based ).
As a base you can use electornic telephone
design . The hybrid transformer , input (microphone)
output (speaker), hook on/off circuits - all are there . You canuse only transistors or simple chips to make it cheap .
Optocoupling also possible - but it will just add coupling circuits to the mentioned above .