Sonic.net

There are services that allow you to buy vanity phone numbers in various area codes - e.g. https://www.numberbarn.com/vanity-phone-numbers

I'm a current sonic subscriber with a sonic-provided number.

Can I buy an arbitrary number and then port it to use as my Sonic phone number? Or do I need to specifically choose a San Francisco area code?

We can only port numbers that are in the same LATA (Click here for wiki page). For example, if you wanted a 707 area code in SF (which is normally 628 or 415) we would be able to port that number in but if you wanted a 323(LA) then we wouldn't be able to port that number.