I have used Zoiper on my Iphone for over a year now. Right after updating to v 3.15, the following problem occurs. When I press the "Call" button on the dialpad after inputting any phone number, the phone hangs. Nothing responds until I double click the home button and swipe away the Zoiper app. Using "history" to dial out also causes hanging. Incoming calls can still be received with audio in both directions with no problem.
Some details: Account is IAX and the phone confirms that the account is registered. 2 phones a 5S and 6S both behaving the same way after v 3.15 update. I have an Iphone SE that still has v 3.14 and that still works perfectly.