Calls going straight to voicemail

Hi.
A couple times recently I needed to call a company's support ppl and when they called back their call went right to my iPhone voicemail it didn't ring or anything. I googled and found out about an iPhone setting called Silence Unknown Callers but I checked and I don't have that on. B'cuz I don't know what number the support ppl will use I can't put them in my Contacts. How can I make sure that any body that calls I can see the call and answer it?