diff --git a/README.md b/README.md
index 4d1b24b..b3a7353 100644
--- a/README.md
+++ b/README.md
@@ -23,7 +23,9 @@ alarmreceiver.conf:
 * eventcmd = /usr/local/bin/asteriskcontactid.pl
 * eventspooldir = /var/spool/asterisk/alarmreceiver
 
-## Recommended FXS Adapter
-The Grandstream HT802 configured for DTMF inband and passthrough is recommended for connecting a Contact ID dialer to Asterisk.
+## FXS Adapter
+This has been tested with a Grandstream HT802 and an Adtran Total Access 904 configured for DTMF inband and passthrough to connect a Contact ID dialer to Asterisk.
 
 * https://www.grandstream.com/products/gateways-and-atas/analog-telephone-adaptors/product/ht802
+
+Some newer Contact ID dialers may be more strict abut the quality of tones generated by the FXS device. In testing the HT802 worked well with a Silent Knight 5700 FACP, but a newer 6820 FACP would not recognize the kissoff tone through the Grandstream. Replacing it with an Adtran Total Access 904 fixed the issue.