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If Caller ID detection is STILL not functional after the hardware modification,
a possible though rarely used solution is to set the timing of certain 
registers.

Robert McKeever (mckeeveb@sfu.ca) writes:
Way back in September, ZyXEL released a note (now disappeared off my drives)
which described the use of the S45 and S46 registers to set timing.

They (wrongly) assumed that the ringing on the GTD-5 exchanges was exactly
1.75s, following by 50ms silence, then data;  for DMS-100 as a 2s - 3.5s ring
interval, followed by 500ms of silence.

The CallerID length depends on the amount of data the exchange transmits
as its message.


    **********************------++++++++++++++++++
               ^             ^            ^
               |             |            |
             Ring         Silence     CallerID Data

I quote from their message:

<< It is difficult for the modem to receive CallerID signals for both exchanges
without configurable parameters.  Two new parameters are implemented for this
purpose, S45 and S46.  They are called "guard time" and "silence interval",
respectively.

The silence interval is the consecutive silence that the modem should detect
after ring on before CallerID signal to be processed.  Distinctive ringing may
be combined with CallerID, and therefore a guard time is needed for the modem
to inhibit the silence detection.

Registers S45 and S46 are in units of 20ms.  Default setttings should be in
the range of S45=100, S46=28 for DMS-100 (Bell, Northern), and possibly
S45=87 and S46=3 for GTD-5 switches. >>

The place this argument falls down, is on the GTD-5 switch, where they
use "flash ringing".  Flash ringing starts at any point in the ring cycle,
which means you can get anywhere from 0.1s up to 2.0s of ringing current
before the 50ms silent period followed by data from the central office switch.

In any event, you now have the parameters for the critical registers
for CallerID, for whatever good it may do you.