Speed dial can be accomplished by creating call rules. They do not need an extension which will take a subscription.
Go to Setup > Call Routing > Outgoing Calls
Create Outgoing Call Rule. See example:
Rule Name and Note:
Name: Speed Dial
Note: Dialing 100 will accomplish the same as dialing long distance call to 1555-555-5555
Pattern to Match:
Number begins with the digits [ 100 ]
The rest of the number must be between [ 0 ] and [ 0 ] digits in length
Before connecting the call, trim [ 3 ] digits from the front, and then prepend the digits [ 15555555555 ]
Call Through:
Primary Call Through Provider: [ set to SIP Providers, IAX Providers, or Channel Groups ]
Rule Final:
Is this rule final? [ YES ]
Allow existing extensions to use this rule [ YES ]
Save Outgoing Call Rule
Move the call rule up right before your 'Internal' Rule
Make sure to reserve 100 for speed dial and to not create SIP extension 100 because this will conflict with call rule.