In this context, "allowed" is customary. The usage is "minimum/maximum digits allowed" or "number of digits allowed"or simply "digits allowed". Some systems allow only a fixed number of digits, others allow a variable/configurable number.

Cisco Unity Express 3.0 GUI Administrator Guide - Windows and ...Specify the call pattern, maximum digits allowed, and minimum digits allowed. ... The Voice Mail menu appears when you click Voice Mail on the Home window. ...
www.cisco.com/en/US/products/sw/voicesw/ps5520/products_adm...

7 Oracle Voicemail & FaxOracle continues to enhance Oracle Voicemail & Fax capabilities with .... when the user enters a password outside of the minimum or maximum digits allowed. ...
download-west.oracle.com/docs/cd/B13866_04/extras.904/b12240/voicemail.htm

[PDF] Hard Drive Voice Mail Installation and Supervisor GuideFile Format: PDF/Adobe Acrobat - View as HTML
1.2 Understanding the DX-80 Hard Drive Voice Mail Product Design. ...... DIGITS ALLOWED WHILE HOLDING. While a caller is in the queue holding for an ...
www.emlinkint.com/library/manuals/comdial/dx80_vm_ref.pdf

[PDF] Norstar Voice Mail Reference GuideFile Format: PDF/Adobe Acrobat - View as HTML
Mailbox. A storage place for voice messages on the Norstar Voice Mail system. Mailbox number length. The number of digits allowed in a mailbox number. ...
www.nev-comm.com/NVM4x_UserGuide.pdf

Note: The use "recogible" surely seems curious since it's usually used for things like retractable awnings, wedding dresses that can bustled, and things that are collapsible, gathered or bunched up in that sense. In input systems, digits are usually "input" and "captured" (rather than "gathered" or "collected", though "gathered" and "collected" are used, too). Since you say that in your context the reference is to personalizing a voice mailbox, "allowed" seems to me to be more appropriate for your case. It typically used to specify the permitted length of IDs, passwords and PIN numbers. (Also certain characters may be "allowed" or "not allowed" -- #, @, etc.)