Configuration Zoiper

Download application from Apple Store.

Download the app from Apple Store.
Click on Plus (+) -> SIP Account -> Manual configuration. Look at the 3 photos below.

Fill your 4Z.COM number ( 41xxxxxxxxx) and service password and sip number. Look photo 2  below.

Slide down and click on Network Settings.

See if STUN is disabled.
MANDATORY DISABLED STUN