Please make sure that the VoIP line you are dialling which belongs to your colleague is recorded against your colleagues contact record inside your CRM account - as shown in the example below:
If you search on the VoIP line number from the Contacts page you will probably find this VoIP line is recorded against another contact (who may have been a previous CRM user) using this VoIP line.
Just remove the VoIP line recorded against the record if the person no longer has the line associated with them.
Now when you dial the VoIP line you will see that it associates the line with the correct contact.