Calls echo audio while calling

What problem did you encounter

Having audio calls with other cellular phones seems to echo back the non-devkit caller voice quieter and a few seconds later.

What is the actual behaviour?

non-devkit caller getting echo of their voice

What is the expected behaviour?

No echo of voice

How to reproduce

1. Call or be called with a devkit and another phone
2. speak into the devkit, all works
3. speak into the other phone, it is heard by the devkit, but also quietly and a couple seconds later echos back whatever was sent to the devkit.

NOTE: Yes it seems strange, this was tested on three different "other" phones with three different numbers to/from the devkit with the same echo result. It is almost as if the other phones audio comes into the devkit and then sends it back quieter to the other phone again.

Which version of Calls did you encounter the bug in?

• I compiled it myself. If you compiled Calls from source please provide the git revision via e.g. by running git log -1 --pretty=oneline and pasting the output below.

• I used the precompiled Debian package (e.g. by running a prebuilt image). Please paste the output of dpkg -s calls below.

purism@pureos:~$ dpkg -s calls                                                  
Package: calls                                                                  
Status: install ok installed                                                    
Priority: optional                                                              
Section: comm                                                                   
Installed-Size: 324                                                             
Maintainer: Bob Ham <rah@settrans.net>                                          
Architecture: arm64                                                             
Version: 0.0.1~158.gbp3160fb                                                    
Depends: libc6 (>= 2.17), libglib2.0-0 (>= 2.43.4), libgsound0 (>= 1.0.1), libgt
k-3-0 (>= 3.19.12), libhandy-0.0-0 (>= 0.0~git20180429), libmm-glib0 (>= 1.5.993
), libpeas-1.0-0 (>= 1.0.0), modemmanager                                       
Description: Make and receive PSTN phone calls                                  
 A GTK+ user interface for PSTN phone calls using oFono.                        
Homepage: https://gitlab.gnome.org/Community/Purism/calls                       

What hardware are you running Calls on?

• amd64 qemu image
• Librem5 devkit
• other (please elaborate)

What modem are you using?

devkit provided SIM7100A