telephone number
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... host. The user part is a user name or a telephone number.
The host part is either a domain name ...
... Internet telephony gateway, the user field
MAY also encode a telephone number using the notation of
telephone-subscriber ...
... telephone-subscriber (Fig. 4). The telephone number is a special
case of a user name and cannot be distinguished by a BNF ...
... BNF. Thus,
a URL parameter, user, is added to distinguish telephone numbers
from user names. The phone identifier ...