SPA2102 doesn't see VMW from SPA9000

Question

Can anyone suggest why the SPA2102 can't detect voice mail waiting from the SPA9000? The SPA9000 shows voice mail waiting for the SPA2102 extension, but the SPA2102 shows status Message Waiting = No under the line status information, and the attached phone gets no message waiting indicator.
The Voice Mail Server field on the Line 1 page of the SPA2102 is set to [email protected]:6060, where the SPA400 providing voice mail is on Line 2 of the SPA9000, the extension number of the SPA2102/Line 1 is 201, and the IP address of the SPA9000 is 198.162.0.1. The phone works fine to make and receive calls via the SPA9000, and it responds correctly with a message waiting indication when the Message Waiting field on the SPA2102 User 1 page is set to Yes manually.  It doesn't make any difference to change the Mailbox Subscribes Expires field to a shorter interval, or to play with the VMWI Ring Policy options.
The SPA9000 shows the voice mail waiting for extension 201 in its Mailbox Status field for Line 2 (the SPA400). Also, the message waiting indicator does work in the expected way for phones attached to the the built-in FXS ports on the SPA9000, although it has a separate Mailbox ID field on the FXS settings page. In other words, the same phone does show the correct message waiting status if attached to the SPA9000 FXS#1 port with the Mailbox ID set to 2201.
Any guesses or suggestions?

Answer 1

Yes, we've read all those links, starting with the early ones where Linksys claimed that the SPA9000 worked with any SIP client, then the ones where they fell back on the stock excuse "it works with the SPA9xx phones", then the ones where they quietly fixed many of the bugs in firmware updates without ever acknowledging that they were bugs.
In our testing we have found that the SPA9000 now actually handles most functions with analog phones attached to its own FXS ports and via other ATAs like the SPA2102.
Remaining bugs:
1. The SPA9000's built-in FXS ports can detect Message Waiting from the SPA9000, but other ATAs like the SPA2102 cannot.
2. Neither the SPA9000's built-in FXS ports or any other ATAs can transfer calls to the SPA400 voicemail due to its quirky non-standard messaging protocol. However, they can call the voice mail manager (just make sure that DTMF is set to INFO+InBand), and the SPA9000 contact list or auto-attendent functions can transfer busy/no-answer calls to voicemail.
3. Group pickup doesn't work.