San Francisco has grown so much that like other big cities, availability of numbers in the 415 area code is depleted. A new code is now available covering the same area: 628. This was introduced in February 2015. (More: San Francisco Chronicle).
There are plenty of 628 numbers available. Other nearby area codes like 510, 650 and 408 are still available too, so these may be a good alternative.
If you really need a 415 number, reach out and we'll try and find one. You can also keep searching in Talkdesk on the add numbers page as they will periodically reappear as numbers are recycled. As there are no new numbers in 415, whichever number you get will have previously been used before. You might prefer to have a new 628 instead.