I think there would be a possible solution for your use case. It is a little more tedious to make, but I think I may just work. I have not tried it myself, so I am not 100% sure that it will do what you want.
If I understand correctly, in a nutshell, you want this call routing flow:
DID → IVR → Subaccount → Cell phone → Voicemail.
Here is how I think this can be configured.
DID → IVR → Call Hunting → Ring Group → Forwarder → Voicemail.
The magic happens in cascading the Call Hunting into the Ring Group as the second option. The Call Hunting gives the ring sequence between the subaccount and the cell phone forwarder while the ring group bring the capability of having the call transferred to a voicemail as the last resort.
Step 1 - Voicemails:
Configure your Voicemails (you should already have this done)
Step 2 - Forwarders
Configure your Forwarders, one for each cell phone (you should also have this done by now)
Step 3 - Ring Groups
For each Forwarder, create a Ring Group which has only one member: the Forwarder to the cell phone. In that ring group set the voicemail to the one you want to send the call to if the cell phone does not answer after the delay.
The use of a Ring Group here is the only way I could find that would allow to make a fallback to the voicemail of your choice.
Step 4 - Call Hunting
For each subaccount, create a Call Hunting with 2 members: The subaccount that will ring first and the Ring Group containing the cell phone forwarder and the voicemail that you want as the fallback. Set the order of the hunting to “Follow order”. You may need to put plenty of time to the Ring Group member because you have to account for the cell phone ringing time and the voicemail recording in the case the forwarder does not answer and the call goes to the voicemail. If you put too little time for the Ring Group, I fear that the system may hang up while the voicemail is playing the announcement or while the caller is recording their message.
Step 5 - IVR
Point each of your IVR options to the corresponding Call Hunting.
Just be careful about the cell phone’s own voicemail. If the delay for the forwarder in the Ring Group is too long, the Cell phone may actually start going to their own voicemail before being cut by the Ring Group delay and go to the VoIP voicemail. This can be weird for the caller having an announcement starting and being cut by another announcement. Also, if the cell phone is not turned on when a call is forwarded to it, the call may go directly to the cell phone’s voicemail and the caller may be forwarded to the intended VoIP.ms voicemail while recording their message on the cell voice voicemail. Unfortunately, you cannot control the cell phone voicemail sequence, unless you configure all your cell phones to never go to their voicemail.
Let me know if this works for you.