I’m not a telco expert; but my understanding is that phone traffic (while architecturally pretty much packet-switched VOIP-esque stuff at this point, line-switched is Dead except at a few points at the margins) doesn’t slosh around quite as much as internet traffic because it’s all quite latency sensitive, and because (even more so historically, and into the present) much of the traffic was essentially internal, subscribers of the same monopoly telco chatting with each other, so telco capacity was built much more around accommodating local traffic with limited volumes of international calls.
I have no reason to suspect that anything actively prevents an international hop on a domestic call; but would suspect that they are less routine than with internet traffic.
The NSA apparently has a cushy little deal with telco operators in one 6-7 countries, or is managing to run one hell of a hack(and a fairly massive data exfiltration operation) without being noticed…
