Registration Phone Number Field Issue

### 1. Reproduction Steps

+ Enrolled a new student.
+ Filled out the registration form with information.
+ Filled phone number field with letters instead of numbers.

### 2. Expected Behavior

+ form would refuse alphabetical data in the phone number field.

### 3. Actual Behavior

+ Form accepted non-numeric data.
+ It registered the student with meaningless phone number info.

### 4. Error Messages / Logs

N/A

### 5. System Report

```
Wordpress
-------------------------------------------

Home Url: http://llms-73.test
Site Url: http://llms-73.test
Login Url: http://llms-73.test/wp-login.php
Version: 5.1.1
Debug Mode: Yes
Debug Log: Yes
Debug Display: Yes
Locale: en_US
Multisite: No
Page For Posts: Not Set
Page On Front: Not Set
Permalink Structure:
Show On Front: posts
Wp Cron: Yes

Settings
-------------------------------------------

Version: 3.30.0
Db Version: 3.30.0
Course Catalog: Course Catalog (#5) [http://llms-73.test/?page_id=5]
Membership Catalog: Membership Catalog (#6) [http://llms-73.test/?page_id=6]
Student Dashboard: Dashboard (#8) [http://llms-73.test/?page_id=8]
Checkout Page: Purchase (#7) [http://llms-73.test/?page_id=7]
Course Catalog Per Page: 9
Course Catalog Sorting: menu_order
Membership Catalog Per Page: 9
Membership Catalog Sorting: menu_order
Site Membership: Not Set
Courses Endpoint: my-courses
Edit Endpoint: edit-account
Lost Password Endpoint: lost-password
Vouchers Endpoint: redeem-voucher
Autogenerate Username: yes
Password Strength Meter: yes
Minimum Password Strength: strong
Terms Required: no
Terms Page: Not Set
Checkout Names: required
Checkout Address: required
Checkout Phone: optional
Checkout Email Confirmation: yes
Open Registration: yes
Registration Names: required
Registration Address: optional
Registration Phone: hidden
Registration Voucher: optional
Registration Email Confirmation: no
Account Names: required
Account Address: required
Account Phone: optional
Account Email Confirmation: yes
Confirmation Endpoint: confirm-payment
Force Ssl Checkout: no
Country: US
Currency: USD
Currency Position: left
Thousand Separator: ,
Decimal Separator: .
Decimals: 2
Trim Zero Decimals: no
Recurring Payments: yes
Email From Address: admin@local.test
Email From Name: llms-73.test
Email Footer Text:
Email Header Image:
Cert Bg Width: 800
Cert Bg Height: 616
Cert Legacy Compat: no

Gateways
-------------------------------------------

Manual: Disabled
Manual Logging:
Manual Order: 1

Server
-------------------------------------------

Mysql Version: 5.5.5
Php Curl: Yes
Php Default Timezone: UTC
Php Fsockopen: Yes
Php Max Input Vars: 1000
Php Max Upload Size: 1 GB
Php Memory Limit: 256M
Php Post Max Size: 1024M
Php Soap: Yes
Php Suhosin: No
Php Time Limt: 30
Php Version: 7.3.3-1+ubuntu14.04.1+deb.sury.org+1
Software: nginx/1.15.10
Wp Memory Limit: 40M

Browser
-------------------------------------------

HTTP USER AGENT: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/72.0.3626.109 Safari/537.36

Theme
-------------------------------------------

Name: Twenty Nineteen
Version: 1.3
Themeuri: https://github.com/WordPress/twentynineteen
Authoruri: https://wordpress.org/
Template:
Child Theme: No
Llms Support: No

Plugins
-------------------------------------------

LifterLMS: 3.30.0

Integrations
-------------------------------------------

BbPress: No
BuddyPress: No

Template Overrides
-------------------------------------------

```

### 6. Browser, Device, and Operating System

Google Chrome Latest, OSX