If I've correctly understood your request, your goal is to get a continuosly ring back tone instead of a disconnect tone after 60 seconds.
Check your Ring Back Tone configuration under Regional parameter menu of the FXS SPA8800.
This is my config: 425@-19;300(1/4/1)
300 is the timer of ring back tone. After 300 seconds the call is rejected.
Check your configuration. I think that you have a value of 60 seconds. If you want set infinit seconds you must use *.
infinit ring back tone: 425@-19;*(1/4/1)
Regards.