0101 numbers are ContactNow internal numbers which are offered to customers to allow them make calls to other queues free of charge. Customers are free to use 0101 numbers as a DDI for inbound queues for the purpose of transferring calls, interacting with other queues etc.
Restrictions are put in place to prevent customers from presenting 0101 numbers as a CLI.
You will not find 0101 in the Caller ID drop down in queues Settings even if the number is unassigned.
This is because if a person dials a 0101 number from an external number, they will be directed to the national emergency services. Therefore OFCOM have stated that we are not to advise customers to set 0101 numbers as a CLI for this reason.