If you have no problems accessing an Apple Modem 300/1200 through AppleLine
but cannot seem to access an Apple Personal Modem, the APM's auto-answer
mode is most likely preventing it from answering.
The auto answer mode is one of the differences between the Apple Modem
300/1200 and the APM. With the Apple Modem 300/1200, the default for the S0
register is S0=1 (that is, "answer the phone on the first ring"). The
default for the APM's S0 register, though, is S0=0 (that is, "do not answer
any ring"). Try connecting a system to the APM and resetting the S0
register to S0=1. Then, without powering down the modem, connect it to the
AppleLine. The modem will now be in auto answer mode, and, unless powered
down or reset, it will answer incoming calls on the first ring.
Another possibility is that you are using the wrong cable. For the APM,
you should be using a 590-0331 (the cable used for connecting an Apple
Personal Modem to an Apple IIe with a Super Serial Card) along with the
modem elinator cable 590-0029.