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18. Phone numbers contain only digits

This one was not obvious, but sure it is possible. DTMF [0] encodes digits 0-9, symbols * and # as well as letters A to D.

[0] https://en.wikipedia.org/wiki/Dual-tone_multi-frequency_sign...

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Okay, both * and # do have meanings in certain areas (and still are), but outside of US military (and this is actually already historical, nowadays priority is linked to the specific handset and some lines are always reserved) Priority A (Flash Override (FO), for Presidential use), Priority B (Flash (F), for breaking events), Priority C (Immediate (I), for urgent but non-life and security-threatening events), and Priority D (Priority (P), officers versus ordinary soldiers) are never used because a) due to the possibility of confusion with numbers like 1-800-ALPHABET and b) AT&T have invented Priority signals specifically for US military's use (although standardised now internationally for some reason: https://www.itu.int/rec/T-REC-Q.955.3-199303-I/e).

If we are talking about phone numbers for humans - maybe true. Phone network routing numbers can and do contain A-E.

I mean, for the purposes of `libphonenumber`, PrioA-D is not used.

Also, as far as I know most things are sent out-of-band anyways, but I wouldn't be surprised if there's some legacy system out there which work only on DTMF.

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