Asterisk phone grandstream gxp2020

Go into the advanced settings of your grandstream to specify the distinctive ring fields 1-3 to use a unique string, in this case CustomStr1

lines similar to the above would go into your extensions.conf, to trigger the approriate distinctive ring field depending on whether the text CustomStr1 was entered as distinctive ring 1, distinctive ring 2, or distintive ring 3 in the advanced settings of the gxp2020 web admin.

The above info is lifted from the Grandstream FAQ as of 2007 August 6th
http://www.grandstream.com/asteriskfaqs.html#Q1