I can make outgoing calls but when I am called the phone doesn't ring it diverts to the answerphone company.
Do you have the sip account configured at multiple clients?
Not all providers allow the use of one account on multiple clients.
How did you configured zoiper?
you actually might have to add a sub account for your DID
I had to create a new DID for my business and 2 sub account for that same #
talk to your voip provider support