I have a similar issue at the moment.
One Huntpilot, Huntlist and Linegroup with 4 members.
3 of these members have mobility configured.
2 of the members with mobility do not have a phone assigned but only have an extension mobility device profile.
When one member of the linegroup:
1) is not logged in with his device profile (EM)
2) has Mobility is disabled
3) noone else is available (hlog or busy)
If you call the huntpilot, the call gets forwarded with "no answer" instead of "busy".