Depending on the country you live in, the "POTS"/PSTN (Public Switched Telephone Network) may have already disappeared or is disappearing (as in all or most calls are now going over packet networks and not dedicated phone lines).
I'll answer your question with regards to the SIP protocol, which is the most commonly used protocol for telephony (as in the one supported by many "modem-routers" you'll get from your ISP, the one offered by countless telecom companies for business lines...).
On leg 1, many equipment simply don't support encryption.
I do not have information about the encryption on leg 2. Interconnection between telcos is often handled thru SS7, which is known to be extremely "trusting and open", leading to many vulnerabilities. That does not bode well.
On leg 3, it's actually the same as leg 1. Just a different customer.
Over the last 15 years I have myself dealt with a few (10+) SIP telcos across several countries. At the moment, I'm "still using 4". Only one offers standard encryption support (as in, following established standards, TLS/SRTP in that instance), with another one offering encryption, with a very specific piece of hardware (that I don't use).
My own view: I assume my communication are intercepted. And that whoever listens to them died of boredom, but that's another issue.