As I understand it, the “UK GDPR” is basically the same thing as the EU’s GDPR. They need to maintain “adequacy” to continue to comply with the UK’s laws and guidelines, so they can’t simply block all non-UK traffic.

You’re also missing the most important part: that modern voice calls have extreme amounts of compression on it. So even if you were able to modulate the connection correctly, it’d be even slower than the slowest POTS dialup.