Actually, I kinda agree with OP but not too low. I've had TMo and have Verizon now since iPhone6 release.
I find VoLTE/Non-VoLTE call volume on TMo a bit lower than their HD Voice on HSPA. But of course, VoLTE is much better and clearer.
With Verizon, when calling a non-VoLTE capable device, it is a bit lower too compared to TMo HD Voice on HSPA, but much clearer than CDMA calls. VoLTE HD Calls on Verizon are comparable to TMo VoLTE HD calls, but better than to TMo HSPA HD calls.
These observations are both with the phone earpiece and the headset.
I find VoLTE/Non-VoLTE call volume on TMo a bit lower than their HD Voice on HSPA. But of course, VoLTE is much better and clearer.
With Verizon, when calling a non-VoLTE capable device, it is a bit lower too compared to TMo HD Voice on HSPA, but much clearer than CDMA calls. VoLTE HD Calls on Verizon are comparable to TMo VoLTE HD calls, but better than to TMo HSPA HD calls.
These observations are both with the phone earpiece and the headset.