This article applies to 123-reg Domain names
The following rules and restrictions are applicable for .tel domain names:
The minimum number of characters allowed in a .tel domain is 3 and the maximum number is 63 not including the .tel suffix.
Every .tel domain name must contain at least one letter from a-z or "A to Z" in standard US ASCII script.
No .tel domain name can begin or end with a hyphen nor can a domain have a hyphen in the 3rd and 4th position.
A .tel domain name may not consist purely of digits.
Telnic also have a list of reserved .tel domain names that are not available for purchase, you can check the reserved list at http://www2.telnic.org/reserved-names.html.