ATH does not work for call hangup.
You need to use AT+CHUP to hangup voice call.
Try to run AT+CVHU? and check its outpup.
0 - ATH can be used to disconnect the voice call.
1- ATH is ignored but OK response is returned.
In our few RUT955 devices, it returns 1 so ATH does not hangup but AT+CHUP works.
Always use AT+CHUP to hangup voice calls. Also please use gsmctl -A before all commands on cli.