Hi Tom,

It looks like support for the 7906 (and 7921) was added in SRST v4.1. Have a look;

The following IP phones are supported by Cisco SRST 3.3:

Cisco Analog Telephone Adaptor (ATA) 186 and Cisco ATA 188 Version 2.16 and higher with Cisco CallManager 3.3 and higher

Cisco SRST supports Cisco ATA 186 and Cisco ATA 188 using Skinny Client Control Protocol (SCCP) for voice calls only

Cisco IP Phone 7902G

Cisco IP Phone 7905G

Cisco IP Phone 7910

Cisco IP Phone 7912G

Cisco IP Phone Expansion Module 7914

Cisco Wireless IP Phone 7920

Cisco IP Conference Station 7935

Cisco IP Conference Station 7936

Cisco IP Phone 7940 and Cisco IP Phone 7940G

Cisco IP Phone 7960 and Cisco IP Phone 7960G

Cisco IP Phone 7970G

Cisco IP Phone 7971G-GE

It also doesn't show the 7906 in here;

http://www.cisco.com/en/US/customer/products/sw/voicesw/ps2169/prod_installation_guide09186a00805f6f1b.html

It does show the 7906 added in SRST 4.1 (See Table 3)

http://www.cisco.com/en/US/products/sw/voicesw/ps2169/products_data_sheet0900aecd805e1e79.html

Hope this helps!

Rob