LCA2019 has it's very own phone system this year.
Phone numbers are 5 digits long.
Modern Android phones have a SIP client built into them.
On iOS you will have to install an app. Reddit has a few suggestions for apps:
Bria and Groundwire seem to be most commonly suggested as they use a push service and don't drain the battery. Both apps are paid apps.
you will receive an envelope inside your delegate pack with your login details and a link to the voip help page (this page)