5 phones is max for ringing current in the area you are Cee's
More likely i'd say was your running whats known as 3 wire jacks without putting the blue wire through, easyest way to over come this is to replace your jacks with 2 wire jacks, easy to tell which is whick , 3 wire jacks have a M or and E on the front , 2 wire jacks have a 2 on the front. the ones with M's will still be ringing but ones with E's probaly not.