It took me a while to understand this question. I believe that the problem is "Dialing the number registered to Company A connects the caller to Company B".
Ultimately, this is an issue that can be resolved with systematic troubleshooting.
1) The caller is making a mistake (dialing the wrong number). Some phone exchange systems may expect particular prefixes or suffixes that require the caller to modify the number they input.
2) The phone provider the caller is using is misdirecting the call somehow (deliberate or accidental). You can rule this out by repeating the call from a different provider (use a difference cell phone company, or a pay phone).
3) Company B does actually own the number being called; the error is in the caller thinking the number belongs to Company A.
I very much doubt that any "call intercept" is being performed here. It's difficult to do unless you have significant access to the telecommunications network - it's the sort of thing that we would expect only government agencies to be capable of doing. Tampering with the phone system is usually illegal in any country, and doing so in this way would immediately lead investigators to their prime suspect.