ATH1 "off-hook" doesn't work

[cbmbas]

tcpser seems to make no difference between ATH0 and ATH1.
ATH0 = Hang up / On-Hook works, while ATH1 = busy / Off-Hook has no effect.
Loglevel 5 output of both commands:

2019-08-25 14:07:23:-1711884544:DEBUG:Evaluating ATH0
2019-08-25 14:07:23:-1711884544:DEBUG:Command: H (72), Flags: 0, index=2, num=0, data=0-0
2019-08-25 14:07:23:-1711884544:INFO:Disconnecting modem
2019-08-25 14:07:23:-1711884544:INFO:Disconnecting
2019-08-25 14:07:23:-1711884544:INFO:Control Lines: DSR:1 DCD:0 CTS:1
2019-08-25 14:07:23:-1711884544:DEBUG:Command: (-1), Flags: -1, index=2, num=0, data=0-0
2019-08-25 14:07:23:-1711884544:DEBUG:Sending OK response to modem
2019-08-25 14:07:26:-1711884544:DEBUG:Data available on serial port
2019-08-25 14:07:26:-1711884544:DEBUG:Read 1 bytes from ip232 socket
2019-08-25 14:07:26:-1711884544:DEBUG:Read 1 bytes from serial port
2019-08-25 14:07:26:-1711884544:DEBUG:Data available on serial port
2019-08-25 14:07:26:-1711884544:DEBUG:Read 1 bytes from ip232 socket
2019-08-25 14:07:26:-1711884544:DEBUG:Read 1 bytes from serial port
2019-08-25 14:07:26:-1711884544:DEBUG:Data available on serial port
2019-08-25 14:07:26:-1711884544:DEBUG:Read 1 bytes from ip232 socket
2019-08-25 14:07:26:-1711884544:DEBUG:Read 1 bytes from serial port
2019-08-25 14:07:27:-1711884544:DEBUG:Data available on serial port
2019-08-25 14:07:27:-1711884544:DEBUG:Read 1 bytes from ip232 socket
2019-08-25 14:07:27:-1711884544:DEBUG:Read 1 bytes from serial port
2019-08-25 14:07:27:-1711884544:DEBUG:Data available on serial port
2019-08-25 14:07:27:-1711884544:DEBUG:Read 1 bytes from ip232 socket
2019-08-25 14:07:27:-1711884544:DEBUG:Read 1 bytes from serial port
2019-08-25 14:07:27:-1711884544:DEBUG:Evaluating ATH1
2019-08-25 14:07:27:-1711884544:DEBUG:Command: H (72), Flags: 0, index=2, num=1, data=0-0
2019-08-25 14:07:27:-1711884544:INFO:Disconnecting modem
2019-08-25 14:07:27:-1711884544:INFO:Disconnecting
2019-08-25 14:07:27:-1711884544:INFO:Control Lines: DSR:1 DCD:0 CTS:1
2019-08-25 14:07:27:-1711884544:DEBUG:Command: (-1), Flags: -1, index=2, num=0, data=0-0
2019-08-25 14:07:27:-1711884544:DEBUG:Sending OK response to modem

"1" after ATH is recognized -> num=1.
On a RS-232 Tester which displays the Lines with LEDs, DCD doesn't change state after ATH1.

I guess it's located in modem_core.c:
case 'H':
if (num == 0) {
mdm_disconnect(cfg);
}
else if (num == 1) {
mdm_answer(cfg); <----
}

This only makes line busy, on a "RING". But I don't know, if that's the right / only Codepart.
SysOps sometimes want to make their Line busy f.e. for BBS maintanance with ATH1, which currently doesn't work.