If you need reliable low latency, as you want for a phone call, you get very little throughput.
Examples of such connections are wifi near the end of the range, or LTE connections with only one signal bar.
In those cases, a speedtest might say you have multiple megabits available, but you probably only have kilobits of bandwidth if you want reliable low latency.
Also, Bufferbloat is usually not (only) caused by you, but by people sharing the same chokepoint as you in either or both directions. But if you're lucky, the router owning the chokepoint has at least some rudimentary per-flow or per-IP fair scheduler, in which case sending less yourself can indeed help.
Still, to have that effect result in a usable data rate of kilobits on a connection that can otherwise push megabits (disregarding queueing delay), the chokepoint would have to be severely overprovisioned and/or extremely poorly scheduled.
Queueing aware scheduling algorithms are much more effective, are readily available in Linux (tc_codel and others), and are slowly making their way into even consumer routers (or at least I hope).