The definition of "isochronous" is that of something that happens at a regular interval. Wikipedia:
In telecommunication, an isochronous signal is a signal in which the
time interval separating any two significant instants is equal to the
unit interval or a multiple of the unit interval. Variations in the
time intervals are constrained within specified limits.
A voice call is isochronous as it is transferring bits of your voice at regular intervals.
The second part of the sentence
with a clock distribution tree radiating from a single, protected cesium reference clock.
seems to indicate that the transport network carrying voice calls relies on a centralised clock. SDH would be an example of a technology that does rely on synchronised clocks and using caesium clocks as a source of that clock is fairly common. A Caesium clock will only give you a frequency, which is what is required for operating an SDH network. GPS could be an alternative source that would also provide absolute time, i.e. date, time of date and so forth. It's not uncommon to use both, with GPS as primary and Caesium as a backup. Using multiple clocks also allows you to detect drift (someone could attempt tampering with GPS). Without accurate clocks you will get frame slips in your SDH network effectively dropping bits in your payload.
Again, as put by Wikipedia:
"Isochronous" is a characteristic of one signal, while "synchronous"
indicates a relationship between two or more signals.
From a technical perspective perhaps the "telephony network" should refer to the transport network that is carrying those calls, in which case it might not be isochronous (SDH definitely isn't) but when just looking at the calls themselves they are isochronous. Regardless of underlying carrier you will have bits coming in at regular intervals, representing the audio stream, thus making the voice calls isochronous.
I agree that the sentence is a bit weird but depending on your interpretation it isn't necessarily incorrect.