Between the phone and the cell-tower relatively secure, from there on to the telcos nearest internet gateway "it depends" and when your packets reach the internet they're on their own (so use protocol that encrypts the packet payloads if you worry about sniffing).

Note that "it depends" can go to ridiculously low values, at one time it was not uncommon to have the (highly directional) microwave links between cell-towers (not all of them have dedicated land lines) run unencrypted and with proper equipment it was easy to sniff all traffick going out of the tower (essentially negating the link-level encryption between the phone and the tower to all but the most casual eavesdropper).