Use pkg_add -i to choose from package flavours.
Package description (pkg_info asterisk) tells us that it can be standard or have IMAP flavour:
Asterisk supports Voice over IP in many protocols, and can
interoperate with almost all standards-based telephony equipment
using relatively inexpensive hardware.
Flavours:
IMAP - use imap for voicemail storage instead of files.
Maintainer: Stuart Henderson
WWW: http://www.asterisk.org/
Installation:
# pkg_add -ivvv asterisk
Ambiguous: choose package for asterisk
a 0:
1: asterisk-10.12.1
2: asterisk-10.12.1-imap
Your choice: 2