With Voxer you can:
Talk, send text and photos - send live or recorded voice, text messages and photos instantly—to one person or an entire group
Listen live or listen later - Listen ‘live’ while someone is speaking and respond immediately or listen later and reply when it’s convenient
Use any network, globally - Voxer works on any cellular or Wifi network around the world
Website: https://www.voxer.com/
On a your / a clients' phone:
Ensure they are on Wifi or have sufficient data
Select 'Sign Up' or 'Log In'. For Sign Up, fill in the details and 'Click Create New Account'
Doing this for a client on their phone: Put the Voxer app onto the home screen of their phone and explain and show them how it works. If you have access to their emails, go in and verify their email address for them.