I need to configure a NetVanta 1638P as a DHCP server and include option 156 in its DHCP Response.

it is possible to add options to your DHCP configuration.

The typical syntax would be option 156 <ascii, hex, or ip>  <value>, so if you are using this option for an IP address setting, use the following as an example:

ip dhcp pool Voice

network 10.100.41.0 255.255.255.0

default-router 10.100.41.1

dns-server 8.8.8.8 4.2.2.2

option 156 ip 10.100.41.200

That is an example with a random network range, but it is correct, and should work for what your are trying to do.

The important part is choosing ascii, hex, or ip based on what type of value you need.

This may be helpful for you as well:  https://supportforums.adtran.com/docs/DOC-1851

Sounds like you putting in a Shoretel phone system. This is the string you'll need: option 156 ascii "ftpservers=X.X.X.X, country=1, language=1, layer2tagging=1, vlanid=2"

You can drop the layer2tagging and vlanid if you're not using vlans. You can add this from the dhcp pool section of the web interface. Go into the pool, go to the 3rd tab on the top called Numbered Options. Put 156 in the number field leave the type to Ascii text and put your sting in the "ASCII Text" field.

Good luck!