Pardon my confusion, but it was originally stated that the SpeedStream 5100 modem worked fine. If it was able to function without various phone-line switching adapters, shouldn't the P645 work without these adapters, also?
One thing that definitely would have helped with the ZyXEL manual was for them to say:
A) If the DSL light is out, check these things.
B) If the DSL light constantly flashes, check these.
c) If the DSL lights steady Green, but there is no connectivity to the ISP, this is what you need to check.