First I would use unlec.com to determine where it is currently allocated. The SPID/OCN tells you who has it. SPID = Service Provider ID; OCN = Operating Company Name.
Then look at the LNP history, which is the history of who and when the number was assigned/re-assigned over the years.
Tell both companies that you will be involving the FCC and try to reach the "porting group" who will be able to fix this. Porting problems happen all the time, even with 99% of ports (that might be an optimistic number) happening in a nearly-automatic fashion. (EDIT: I mean the porting group at each company, not the FCC).
Then look at the LNP history, which is the history of who and when the number was assigned/re-assigned over the years.
Tell both companies that you will be involving the FCC and try to reach the "porting group" who will be able to fix this. Porting problems happen all the time, even with 99% of ports (that might be an optimistic number) happening in a nearly-automatic fashion. (EDIT: I mean the porting group at each company, not the FCC).