I use a Intel modem iPhone X (that means VoLTE only) and live in a high rise in Jersey City.
I get constant full bar signal, and speed test almost always gives me 10/0.42 Mbps.
When I place or pick up a call, I can hear the other party clearly, but almost always the other party hears me tapping in and out.
And because the "signal strength" is strong, Wi-Fi calling auto-switching doesn't kick in. So let's not discuss it here.
Is there any way to fix this?
An upload of 0.42Mbps suggests a weak return path to the tower. Unfortunately, field test mode is broken on the Intel model of the iPhone X so we can't use that to get some signal information. Do you have a Qualcomm device handy which you can toss your SIM into, that supports Bands 2, 4, and 13?