Hi, I don't know if this answers your question but it may help:
I use a D-Link DWR-933 mobile hotspot (4G/LTE) with a Sunrise subscription and pass VOIP calls using the Realme 14 Pro+ connected to Switzernet. I'm based in Winterthur where Sunrise has a less than 7 mW antenna in the Altstadt. The bandwidth varies from 3 MBit/s to 45 MBit/s. It depends on how many people are on the street and how close to the window my mobile hotspot is located. This Friday morning, with the mobile hotspot against the street window at a distance of about 100 m from the Sunrise antenna, the cnLab speedtest shows download average 21 MBit/s, Fastest 35 MBit/s, and Upload average 23 MBit/s and fastest 32 MBit/s.
I can pass phone calls without problem. However, if I download files using my laptop during the call, then there will be a short cut in the sound of the call but the call will not be disconnected. I can also make video confcalls over Google, Teams, Zoom, Signal, WhatsApp without problems over phone or laptop. But remember, I'm only 100 m from the Sunrise antenna. If I go to Marktgasse, for example to the Sunrise store there, then the connection is not great. The people who work there at the Sunrise shop are embarrassed about it. Despite many requests, Sunrise has made no effort at all over the years to improve the situation in Winterthur. In comparison, Swisscom has a 500 mW antenna. That's probably much better although I have not tried it.
In Zurich the situation with Sunrise is much better. Sunrise has 500 mW antennas installed so the bandwidth is much better.
So my recommendation before using the Sunrise network is to go to
https://www.bakom.admin.ch/de...and check how far you are from a Sunrise antenna, how many rows of houses are between you and the nearest antenna, and how powerful the GSM antenna is. I have only one row of houses so it's almost direct line-of-sight to the 7mW antenna. When I'm in Zurich, the 500 mW antennas mean zero questions or problems.
So if you see that there's only a 7 mW Sunrise antenna that is, say, 300 m away from where you live, I'd say go for Swisscom or a provider that uses the Swisscom network.
If you live in the Zurich area, I'll gladly come over to do a cnLab speedtest for comparison. However, one should remember that a speedtest done outside on the street is much better than inside a house.