I'm strongly against this approach as I described above. From point of theory
and URI handling, <sip:address@hidden> is not equaled to <tel:N> in general.
Anybody
is free to register <sip:address@hidden> on his registrar. Thus embeding
telephone
number into sip: URI is specific to local SIP routing policy and no obligation
in this direction should bee assumed. Also public phone numbers (E.164) are
(usually) maintained by authorites different from local SIP operator,
therefore overloading sip: namespace is wrong way if one recalls SIP is not
about numbers only.
Be ware that SIP protocol allows to INVITE any URI including tel: URI.