Yes the inbound voice menu can be customized by edit the VoIP modules voicexml file.
The default location for inbound voice XMLs is ...Enterprise Alert\VoIPModule\voiceXML. You can add new XMLs to this folder that will be accessible from the Web Portal. A best practice here is to not alter the default files but to make copies and alter the copies instead.
The most common request to change are the DTMF inputs, because of "fat fingering". To do that you will have to change the texts as well as the expected DTMF inputs. The function you are looking for from inside of the voicexml file is:
<menu id="mainMenu" dtmf="true">
<prompt>
Hello, this is Enterprise Alert. To close an alert, press 1. To enter an alarm code, press 2. To leave a voice message, press 3. To connect to an on-call person of a team, press 4.
</prompt>
<choice dtmf="1" next="#formGetTicketID" />
<choice dtmf="2" next="#formRequestAlertCode" />
<choice dtmf="3" next="#formRecordVoiceMsg" />
<choice dtmf="4" next="#formRequestTeamCodeAndTransferToOnCallPerson" />
<noinput>