His rooming house wasn't known by anyone that he knew. Not even Marina. It has been shown that some group did know about it. Most likely it was the people that he was working for.
Ruth Paine had the phone number for the rooming house. Ruth Paine was an FBI informer ( by her own admission to the WC) If Ruth gave the phone number to her FBI contact, they could have located that telephone in five minutes....
And I strongly suspect that the FBI did in fact know about Lee Oswald having a room at 1026 N. Beckley.