Apple ID - Verification calls to landline not working

Recently my son's Mac Book Air died (he is 10 and part of my family sharing). When I try to set his iCloud account up on a new computer the verification code is being send to the dead computer. His trusted number is a landline that cannot receive texts. For some reason verification calls are not working with this number (it just doesn't ring). See images below. Trusted number is correct.

Any ideas? I have contacted support, tried to change trusted number to my mobile. But seem to be in a verification 'Groundhog Day' loop! Day 10+.

ID and password are both correct. Only verification required.