Thanks for contacting support.
We are aware of some devices affected, where the sound output locks to speaker.
Our devs are working on a fix for this.
As a workaround, if you're on the Standard plan, please try going to More tab, set Make Calls & Receive Calls to 'Mobile Network'.
If you're on our Lite plan, then please get in touch via email to 'firstname.lastname@example.org' with your registered number and we'll be able to switch your account to Standard level whilst we work on getting a release to fix this issue.