As of #718 , we now support phone numbers in FedCM. However, we don't require any specific format.

In w3ctag/design-reviews#1074 (comment) the TAG wrote:

We also note that the tel autocomplete field describes a canonical format of "ASCII digits and U+0020 SPACE characters, prefixed by a U+002B PLUS SIGN character (+)" (e.g. "+1 617 253 5702"), while the OIDC phone_number field recommends to use the E.164 format (e.g. "+1 (425) 555-1212"). Your eventual spec should say what's expected and whether and how UAs are supposed to convert them.

We should decide on something here.