Also has come up in https://github.com/metanorma/metanorma-nist/issues/277:

We have (for reasons of legacy) a type on phone, to allow us to differentiate fax numbers from normal phone numbers:

phone = element phone { 
  attribute type { text }?,
  text }

This is not captured in relaton-bib currently: it gives no way of differentiating phones from fax numbers.