Altought  the SIP Phone is well provisioned, it means from the PC phone appears as connected,  when we try to make or answer a call  an AoR error is shown and the call is dropped.

Furthermore, when we try to restart the phone from PureCloud the phone is shown as no connected and obviously restart fails.

Any idea to solve this issue?

Think we have got similar issue and the only way for us to solve it is to close the softphone, start it again and provision once more. 

Thank you, I have already done but the problem still present.

It seems is something common with all configured SoftPhones, like Phone Trunk or Base Phone configuration.