Since firmware version we inhibit (by default) the option to insert letters in the dial screen as you can see in the following image:

In order to restore this possibility you can turn off the settingĀ block_url_dialing

You can perform this in three ways:

  1. Throw the Web User Interface (Advanced - Behavior (Tab) - Phone Behavior (Section): Block URL Dialing)
  2. Via HTTP Request
  3. Via provisioning

Once you have turn Block url dialing to off, you can enter letters again in the dial screen:

Note: you have to change the input keyboard from "123" to "abc" pressing the first contextual key button: