With over 250 million units sold till now, the world's best selling phone till date is the Nokia 1100. The rugged 1100 was the most popular mobile phone in the industry of its time when it was released in 2003. And, without surprise, it still holds the record for the top-selling electronic gadget ever.
The Nokia 1100 became so popular that even its successor, Nokia 1110 basked in its predecessor's glory at the time of release in 2005 which helped Nokia sell over 250 million 1110 handsets.
Nokia might be a struggling mobile phone manufacturer right now but back in the late 90s and early 2000s, it was the largest manufacturer of mobile phones. The aging Symbian OS later became the cause of its decline and recently after accepting Microsoft's Windows Phone OS as the platform for its high-end phone, Nokia sold its entire mobile phone division to Microsoft for $7.2 billion.
The Wikipedia's page of "best-selling mobile phones" shows the dominance of Nokia around 10 years ago reflected by the number of sales of its mobile phones. In fact, seven of the highest selling mobile phones ever are all from Nokia.
The other six highest selling mobile phones ever, after Nokia 1100, are also from Nokia: 1110,1200, 5230, 3210, 6600 and 2600. Nokia has sold well over a billion phones of just these.