Unlimited? The limitation on public part is 20 digits.
That's just a matter of programming these days.
On the Internet these days, it is a matter of hardware.
Ad hoc extension beyond hardware supported length at that time will fatally hurt performance.
What hardware limits numbers to 20 digits today?
On psuedo packet network, such as X.25 or ATM, with full of connection, packets are forwarded by hardware with short connection ids where e.164 numbers are used at the time of complex signalling processed by software.
Masataka Ohta
