Hunt group calls will not forward off-site, but there is an easy work around to accomplish what you want to do.
Point the DDI directly to the receptionist's phone. Create an immediate forward path to the hunt group for Call Routing Table only. Create another DND forward path for Call Routing Table only to a phantom which is forwarded to the moblile phone, make sure appears first on the forward path list for the reception phone.
When the phone is not in DND, DDI calls will ring to the hunt group as usual. When the phone is placed into DND, DDI calls will route to the mobile phone. Flag those forward paths as day mode only, so your night mode will continue to work as usual.