▁ ▁This ▁helps ▁to ▁get ▁the ▁critical ▁mass ▁of ▁implementation ▁to ▁make ▁this ▁method ▁the ▁de ▁fact o ▁standard . ▁ ▁To ▁illustrate ▁this ▁principle ▁with ▁an ▁example , ▁the ▁phone ▁sends ▁a ▁call ▁to ▁the ▁proxy . ▁By ▁using ▁the ▁si ps ▁scheme , ▁it ▁indicates ▁that ▁the ▁call ▁must ▁be ▁made ▁secure . ▁The ▁key ▁is ▁base - 6 4 ▁encoded ▁in ▁the ▁SD P ▁attachment . ▁▁ ▁IN V ITE ▁si ps : * 9 7 @ iet f . org ; user = phone ▁S IP / 2 . 0 ▁ ▁Via : ▁S IP / 2 . 0 / T LS ▁ 1 7 2 . 2 0 . 2 5 . 1 0 0 : 2 0 4 9 ; branch = z 9 h G 4 b K - s 5 kc qq 8 jq j v 3 ; r port ▁ ▁From : ▁" 1 2 3 " ▁< si ps : 1 2 3 @ iet f . org >; tag = m og k xs r hm 4 ▁ ▁To : ▁< si ps : * 9 7 @ iet f . org ; user = phone > ▁ ▁Call - ID : ▁ 3 c 2 6 9 2 4 7 a 1 2 2 - f 0 ee 6 w cr v kc q @ s nom 3 6 0 - 0 0 0 4 1 3 2 3 0 A 0 7 ▁ ▁CS eq : ▁ 1 ▁IN V