Been a while since I used a Grandstream phone and so I just did a bit of research and I can confirm that the Grandstream uses the following syntax:
(Frequencies are in Hz and cadence on and off are in 10ms)
ON is the period of ringing (“On time” in ‘ms’) while OFF is the period of silence. To set a continuous ring, OFF should be zero. Otherwise it will ring ON ms and a pause of OFF ms and then repeat the pattern. Up to three cadences are supported.
http://www.grandstream.com/sites/defaul ... _guide.pdf
Also see: viewtopic.php?f=47&t=1061#p4506