Saying Salaam to Non Muslim, Ignorant or unknown person

بِسۡمِ ٱللَّهِ ٱلرَّحۡمَٰنِ ٱلرَّحِيمِ

All Praise and Thanks be To ALLAH Azwajal
Abu Huraira reported Allah's Messenger (ﷺ) as saying:
Do not greet the Jews and the Christians before they greet you and when you meet any one of them on the roads force him to go to the narrowest part of it.
Sahih Muslim 2167 a (http://sunnah.com/muslim/39/16)

But when it it is necessary to initiate like doing Dawa or Writing letter say "Salamu AAala mani ittabaAAa alhuda" [Peace will be upon him who follows the guidance!]"

[20:47] "So go you both to him, and say: 'Verily, we are Messengers of your Lord, so let the Children of Israel go with us, and torment them not; indeed, we have come with a sign from your Lord! And peace will be upon him who follows the guidance!

Because of this, when the Messenger of Allah (ﷺ) wrote a letter to Heraclius, the emperor of Rome, beginning with "Peace will be upon him who follows the guidance!"
http://en.wikipedia.org/wiki/Muhammad%27s_letters_to_the_Heads-of-State
http://www.qtafsir.com/index.php?option=com_content&task=view&id=921&Itemid=74

Replying to salaam
Anas reported that the Companions of Allah's Apostle (ﷺ) said to him:
The People. of the Book offer us salutations (by saying as-Salamu- 'Alaikum). How should we reciprocate them? Thereupon he said: Say: Wa 'Alaikum (and upon you too).
[Sahih Muslim 2163 b]

Saying salaam to the Ignorant:
وَعِبَادُ الرَّحْمَـنِ الَّذِينَ يَمْشُونَ عَلَى الاٌّرْضِ هَوْناً وَإِذَا خَاطَبَهُمُ الجَـهِلُونَ قَالُواْ سَلاَماً
[25:63] And the servants of the Most Merciful are those who walk upon the earth easily, and when the ignorant address them [harshly], they say [words of] peace

 وَإِذَا سَمِعُواْ اللَّغْوَ أَعْرَضُواْ عَنْهُ وَقَالُواْ لَنَآ أَعْمَـلُنَا وَلَكُمْ أَعْمَـلُكُمْ سَلَـمٌ عَلَيْكُمْ لاَ نَبْتَغِى الْجَـهِلِينَ
[28:55] And when they hear Al-Laghw (dirty, false, evil vain talk), they withdraw from it and say: "To us our deeds, and to you your deeds. Peace be to you. We seek not (the way of) the ignorant."

Salaam to Unknown person, like in telephone
 إِذْ دَخَلُواْ عَلَيْهِ فَقَالُواْ سَلَـماً قَالَ سَلَـمٌ قَوْمٌ مُّنكَرُونَ
[51: 24-25] Has the story reached you, of the honoured guests [three angels; Jibrâîl (Gabriel) along with another two] of Ibrâhîm (Abraham)? When they came in to him and said: "Salâm (peace be upon you)!" He answered: "Salâm (peace be upon you)," and said: "You are a people unknown to me."

Saying salaam to someone unknown on the telephone by Shaykh Ibn Baaz
http://fatwa-online.com/fataawa/miscellaneous/miscellaneous/0060518.htm

All Praise is due to Allaah and may Allaah send His Most Perfect Peace and Blessings upon our Prophet Muhammad, his family and his Companions.