wifi calling issue at oversea

[WIFICALLING]

wifi calling issue at oversea

I set up and activcated my wifi calling on iphone (VERION PREPAID) before leavening USA

here is the details and problems now

1) I can receive and send text without a problem now in oversea

2) I can call my family and other non 866 or 800 number now

but

3) I can not receive calls from my freidns and family in USA. they system said there are restiction on this line

4) I can not call 800 or 866 number to usa

may I know to fix it?

Thank you