Softphone stopped working on Iphone

I have a co-worker who has the application Zoiper (softphone) on her Iphone- it worked perfectly fine for a week and now it keeps doing an error that is related to networking setting.

We checked everything, did what the support of the application told us to do, and nothing worked.

We haven't changed the network settings in the organization, and users who have Androids are able to use the application.

I was wondering if maybe Apple changed the network setting on the Iphone that might be blocking the application from working.

Thanks for the help.