If you can't log in to the Call Utils with your Salesforce account, please check following:

• Log in at your desktop PC - if this doesn't work, contact your administrator because it must work!
• Log in with official Salesforce mobile application from Google Play. If this doesn't work, contact your administrator.
• If login doesn't work, you can try to create new Development Edition org instance with a new user and login there. Don't use your company sandbox. If you still can't log in, there must be any problem with your device, internet connection, etc.
• Check (or contact your administrator) if you have permission "API Enabled". It's enabled by default on every standard profile.
• If you have "API Enabled" permission from your profile or permission set, verify (or contact your admin) if the Call Utils for Salesforce is the Connected App (see this manual) which is not disabled and you have no restriction like end user.
• Uninstall and install again the app. Try this whole troubleshooting again.
• Contact your administrator and / or contact us with detail description (what did you try, what is your device, do you have a internet connection to Salesforce...)