Operating system name and version: Centos 7
Version of smsd: 3.1.15
Smsd installed from: package repository..
Name and model of a modem / phone: Huawei K3765
Interface: USB

Using options

phonecalls = clip hangup_incoming_call = yes

the call is redirected to the voice mailbox.

is it possible to go off-hook for a few seconds, therefore accepting the call, and then terminate the call ?

Version 3.1.15 cannot answer calls. 3.1.23 has a setting answer_incoming_call = <number of seconds> which does the job.

It also has an option to run answer_talker program with answer_talker_args arguments containing the number which is calling and pre-defined parameters. If an audio interface is available for the modem, this feature can say something to the caller, or just play some nice audio. After the program ends, hangup is done and smsd continues.

Currently I'm not sure when 3.1.23 will be released.

Good news (regardless of the last sentence of course).

Any news about 3.1.23 ?

I'm dying to have the answer option.