Add Phone Number to Field Types (#351)

* Add phone number to Field Types

Add phone number to field types
Update phone_number field in system_information.json
Update example

* Update gbfs.md

* Expand phone_number field definition

Add link to E.164 documentation to phone_number field in system_information.json

Co-authored-by: Mitch Vars <mitch@mobilitydata.org>

gbfs.md

@@ -207,6 +207,7 @@ Example: The `rental_methods` field contains values `creditcard`, `paypass`, etc
 * Non-negative Float - A 32-bit floating point number greater than or equal to 0.
 * Non-negative Integer - An integer greater than or equal to 0.
 * Object - A JSON element consisting of key-value pairs (fields).
+* Phone Number - Phone number in [E.164](https://www.itu.int/rec/T-REC-E.164-201011-I/en) format. The phone number MUST start with a "+". The characters following the "+" MUST be integers and MUST NOT contain any hyphens, spaces or parentheses.
 * String - Can only contain text. Strings MUST NOT contain any formatting codes (including HTML) other than newlines.
 * Time - Service time in the HH:MM:SS format for the time zone indicated in `system_information.json` (00:00:00 - 47:59:59). Time can stretch up to one additional day in the future to accommodate situations where, for example, a system was open from 11:30pm - 11pm the next day (23:30:00-47:00:00).
 * Timestamp - Timestamp fields MUST be represented as integers in POSIX time (representing the number of seconds since January 1st 1970 00:00:00 UTC).
@@ -351,7 +352,7 @@ Field Name | REQUIRED | Type | Defines
 `url` | OPTIONAL | URL | The URL of the vehicle share system.
 `purchase_url` | OPTIONAL | URL | URL where a customer can purchase a membership.
 `start_date` | OPTIONAL | Date | Date that the system began operations.
-`phone_number` | OPTIONAL | Phone Number | This OPTIONAL field SHOULD contain a single voice telephone number for the specified system’s customer service department. It can and SHOULD contain punctuation marks to group the digits of the number. Dialable text (for example, Capital Bikeshare’s "877-430-BIKE") is permitted, but the field MUST NOT contain any other descriptive text.
+`phone_number` | OPTIONAL | Phone Number | This OPTIONAL field SHOULD contain a single voice telephone number for the specified system’s customer service department. MUST be in [E.164](https://www.itu.int/rec/T-REC-E.164-201011-I/en) format as defined in [Field Types](#field-types). 
 `email` | OPTIONAL | Email | This OPTIONAL field SHOULD contain a single contact email address actively monitored by the operator’s customer service department. This email address SHOULD be a direct contact point where riders can reach a customer service representative.
 `feed_contact_email` <br/>*(added in v1.1)* | OPTIONAL | Email | This OPTIONAL field SHOULD contain a single contact email for feed consumers to report technical issues with the feed.
 `timezone` | Yes | Timezone | The time zone where the system is located.
@@ -395,7 +396,7 @@ Field Name | REQUIRED | Type | Defines
     "url": "https://www.example.com",
     "purchase_url": "https://www.example.com",
     "start_date": "2010-06-10",
-    "phone_number": "1-800-555-1234",
+    "phone_number": "+18005551234",
     "email": "customerservice@example.com",
     "feed_contact_email": "datafeed@example.com",
     "timezone": "America/Chicago",