A) The issue may be caused by source conflict （One known source that may create such conflict is: http://repo666.ultrasn0w.com）. The conflicting software source should appear in the error message. Removal of conflicting sources should allow successful installation for the OSCall client.
B) Try removing and re-adding the OSCall source on Cydia.
C) Sometimes the jailbreak process may affect the installation of OSCall client. You can try jailbreak your iOS device again and reinstall OSCall client.