How do I fix the call forwarding spinning wheel issue on iPhone?

my friend has the call forwarding spinning wheel. we have tried to reset factory settings, looked at multiple other things in phone settings and we have tried everything it still doesn't accept phone calls and iPhone messages etc… and when I try to call her phone it doesn’t call at all, then goes to voicemail. Please help us! Thank you in advance :)

[Re-Titled by Moderator]