Get NetworkManager to takeover control of /etc/resolv.conf
systemctl disable systemd-resolved
systemctl mask systemd-resolved
to get NetworkManager to take over control.
* Voipfone Softphone (Android) deserves a mention. It needs an account with voipfone.co.uk, but you can signup and try for free. Their softphone app works out of the box for calling SIP URIs - provided they don't have numeric usernames: so you can call AAISP's office (e.g. firstname.lastname@example.org) but you can't call sip:<number>@host (it'll call the number direct instead).