MTE Change Default External Caller ID

I have been trying to change the Default External Caller ID but it is a no go it only works if i set Allow tenant to manage Caller IDs to no

the issue is that a customer had given his cell number to be as the original caller ID, so when he forwards the calls to his cell without using Show originals callers caller ID it goes straight to his cellphone voice mail asking for the password (since the cell phone things he is checking VM

any solutions