The Grandstream HandTones have an issue where Asterisk thinks the device is busy after it sits for awhile. Calling the extension programmed for the device will send your call to voicemail. Picking up the handset will result in no dial tone. However if you hangup and pickup the handset again you will get dial tone. Calling the extension will now ring the phone … for awhile, and then the pattern repeats.

Here is a work around for this.
1. Set the Grandstream to NOT register for any extensions (FXS and/or FXO ports)
2. Set the sip.conf entry for the Grandstream to use a ‘host=ip address’ instead of ‘host=dynamic’. (I.e. host=192.168.0.2)

If you are using FreePBX then do not touch the sip.conf file. Instead bring up the extension associated with the Grandstream device and change the “Host” field from dynamic to the IP address of the Grandstream.

Of course you will want to use DHCP reservations for the Grandstream or assign a static IP address so that the IP address does not change.

I can’t explain why Asterisk and Grandstream don’t play nice together when Asterisk is set to use a dynamic address. All that I know is that everything works perfectly when you assign the IP address. Maybe someone else can shed some light on this phenomenon.