LCA2019 has its very own phone system this year.
Phone numbers are 5 digits long.
Modern Android phones have a SIP client built into them.
CSipSimple is a reasonable free app for Android.
On iOS you will have to install an app. Reddit has a few suggestions for apps:
you will receive an envelope inside your delegate pack with your login details and a link to the voip help page (this page).
VOIP server (or domain) IP: 126.96.36.199 Login: see your envelope PW: see your envelope