Saying salaam to someone unknown on the telephone
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Question: What is the ruling regarding saying salaam (the saying of "as-salaamu
'alaykum") to someone on the telephone if you do not know whether they are Muslim or not?
The ruling regarding that is the (same) ruling of when you meet (someone) and if you know him to be a kaafir then he is not to be greeted with the salaam, however if you do not know, then there is no prohibition in that.
And with Allaah lies (all) the success.
Shaykh Ibn Baaz
Majmoo' Fataawa wa Maqaalaat Mutanawwi'ah - Volume 6, Page 366