If you want to talk to someone on Mastodon without giving them your phone number, you might want to look at Timurs Web Call, which now supports calls using your (and their) Mastodon account names. It also supports Mastodon notification when someone calls.
https://timur.mobi/webcall/mastodon/
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@rudolf Webcall package...
●Didn't work for Android 4.4
●Didn't say min Android requirement...
Have sent this to them but who knows by email...
●Would be good if they were on Mastodon...
maybe one of these is right...
@timur@norden.social
@Timur@mastodon.green
@timurzanagar
@timur@discuss.systems
@rudolf Yup. And do accept my follow request!
@freeschool
Minimal is Android 5.0.