Failover dialplan FreePBX

I have FreePBX has inbound trunks from providers and outbound trunks to customers. I have one customer that if his trunk is down it would dial his cellphone. This should be a simple custom dial plan but I'm sure there is someone out there that can have it done in less time that I can figure out. This is FreePBX distro so any custom asterisk configs must be put in the right place so GUI will not over write them. There are multiple customers on this system so this must be for this customer only. Thanks for looking at my project.