In many areas of the world you can phone switch carriers but keep your phone number. This is known as phone number portability. Caller id and cell phones have encouraged many people to stop memorizing phone numbers, because simply by hitting a button, a person can connect to someone who recently called. 

It is easy to see that some phone numbers are easier to remember than others. In May of 2006 the most expensive phone number was sold in a charity auction in Doha, Qatar.

The expensive phone number 666 6666 reportedly sold for 10m Qatari riyals or £1.5m, or around $2,750,000 US. In the West some consider 6 the imperfect number, but this is apparently not the consensus in Qatar.