We have 4, AT&T 4 line corded phones, model 954. I can't seem to find the owner's manual online, however we are having a problem that could be easily fixed. When someone is on line 1, we cannot hear line 2 ringing on any phone. We've tried the volume and they are all at full volume. Any suggestions? We only have 2 lines that we use. Our email address is register@northpointedance.com. Our phone number is 740-548-4600. Thank You!!