Acrobits' Softphone App

I configured this Acrobits SIP app on my iPhone 6+ and have been successfully using it for 8 months. Three days ago suddenly things changed and now incoming calls work properly, however I receive na “error” with every attempt I try to make an outgoing call. I sent my logs to Acrobits but they said they can try to help must first now why the calls became rejected by FlyNumber. Would you please have any recommendation to solve this problem?
Thank you