When you search in the CDR page, you would find the interesting phenomenon that the outbound call via analog line always show answered. It's hard for accurate billing.
For Asterisk it is hard to identify when it detects the far end has answered the call. As there is no indication typically given when the far end answers only when they hangup. So it is not the PBX issue. When they hangup you will get a CPC disconnect which is like a 500ms battery pulse.
The exception is that when you have polarity service enabled by the line provider. Then you can have noticeable signal of the call state. Polarity is signal of answer or hangup which is also a pulse which sounds like a "pa" sound.
The polarity settings on S-Series PBX.
(Note that if your line doesn't have the polarity service, do NOT check the option in above figure, or it might cause abnormal call issue for your PBX.)
However, the polarity service might charge extra fee that not everyone wants to use it.
There is alternative solution is that avoiding to use analog line in PBX when you need accurate billing.