There is a number assigned to telephones called a ringer equivalency number. This is a rating or threshold above which your telephones may either stop ringing or ring intermittantly. You add up the numbers of all the phones on a "line" and if they exceed the line's capacity (a pots line is 4 or 5).
Example is 4 telephones at .9 each x 4 = 3.6 passes,
5 telephones at .9 each x 5 = 4.5 could be a problem.
Of course this will require troubleshooting. Good luck.