As ckleea C-Hing said, it is not suggested to do so. I don't think they can help switching back one's telephone number to fixed line operator when they want to.
-- Got SIP response 500 "Server Internal Error" back from 202.x.x.x:5060
-- SIP/cmphone-0000001f is circuit-busy
== Everyone is busy/congested at this time (1:0/1/0)