Only some VOIP calls are routed over the internet. Most calls, while digital, are still routed over the proprietary networks owned & operated by the major telcos.

The internet is a packet switched network, which means data is sent in packets, and it’s possible for packets to end up at their destination out of order. Two packets sent from the same starting point to the destination could theoretically go over completely different routes due to congestion, etc. The destination is responsible for putting the packets back together properly. Packets can also get delayed if other higher priority packets come along. It’s for reasons like these that both voice & video on the internet can occasionally freeze, stutter, etc. Granted the capacity & reliability of the internet has improved greatly over time so these things happen less and less often. But the fact still remains that a packet switched network isn’t optimal for real time communication.

Telephone networks on the other hand are circuit switched networks. When you are talking to somebody on a telephone then there is a dedicated circuit path between you and the other person. Each piece of the path between the two of you has a hard limit of the number of simultaneous calls it can handle, which ensures it always has the capacity to serve your particular call. If a circuit between two points is maxed out then the telephone exchange may try to route your call via a different path, or you may just end up with a busy signal.

Packet switched networks also don’t have those hard limits that circuit switched networks do. So packet switched networks can get overwhelmed (think DoS attacks) which can also lead to outages.