Terrible sound quality during voice call when roaming - tested on multiple phones

REPRODUCIBILITY (% or how often): Always
BUILD ID = OS VERSION (Settings > About product): 4.1.0.24
HARDWARE (XA2, X10, X10 II, …): Xperia 10 II dual SIM
UI LANGUAGE: English

DESCRIPTION:

The sound quality when calling or receiving calls on roaming is very, very poor. Sometimes so weak that you cannot hear the other person. Sometimes with so much noise that you can not hear for that reason.
(And sometimes the sound quality is good for half a minute to 1 minute, but never for longer then that.)

This is when not using a headset, but using either the loudspeaker, or using it without the loud speaker.

The sound quality with a wired headset is quite OK - but has a noise to it.

The sound quality using Bluetooth is absolutely superb. Best of all phones I have used.

When not on roaming the sound quality is excellent.

I have tested this on 2 x Xperia 10 II dual sim white, european version and 1 x Xperia 10 II dual sim blue, Hong Kong version.

All with the same result.

What steps have you taken to rule out that it is a problem with the network?

With my Xperia 10 plus dual sim sailfish there is no problem. Neither is there any problems with my Huawei Mate 30 and my LG.

So it is probably a combination of Sailfish, Xperia 10 II dual sim and the network that causes the issue.

My sim card is from Telenor. (Norway)

Roaming is mainly with Vodaphone, but I have forced it to use other local providers, with the same result.

We have the same problems with 3 sim cards from Telenor and 3 x Xperia 10 II.

After testing for over an hour I can add the following:

It is when it is a phone call between 2 phones from the same mobile operator that it does not work.

Calling from my wife’s Telenor to my Telenor - does not work. (Both on phones on roaming.)
Same if calling from my son’s Telenor to another Telenor. (On roaming.)

Telenor to local operator - no problem.

Telenor to other operator - no problem.