There is no technical reason why this should be the case, we test the system regularly, and can confirm that other accounts are able to call each other correctly.
You can check if the calls are getting through to your account by looking at the inbound and outbound Call history in the TMS https://tms.freshphone.co.za
Other than that, also check that Zoiper is up to date, and that its the front most app.
Remember that if you have not changed the settings - Zoiper may not be active in the background - please check the settings on the phone as well as the system settings. A good test would be to have both your phones unlocked, and with Zoiper the font most app - then try call each other. (Zoiper does not support push notification)