How do I restore the phone icon on my iPad?

What icon exactly?

By default the iPad does not have a Phone app, so no specific icon for it would exist.

It has a FaceTime app who's icon looks like this:

If you downloaded an app for a 3rd party phone service and it was deleted, you would need to redownload it from the App Store again.

click here ➜ Redownload apps and games from Apple - Apple Support

Otherwise, check the App Library and see if the icon is there.

click here ➜ Find and use your apps in App Library on iPad - Apple Support

You can't lose something that you've never had. iPad has never had a Phone App icon - quite simply because an iPad is not, and can never be, a standalone substitute for an iPhone. An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or Messaging features. An iPad with Cellular connectivity alone can only access IP-data services.

However, when coupled with an iPhone, the capabilities of iPad significantly broaden to include access to Cellular Voice and Messaging services of the associated iPhone - relayed to the iPad over WiFi using Apple’s Continuity features. For the most part, an iPad using Continuity is able to access Cellular calling features, of the iPhone, when both the iPad an iPhone are in close physical proximity and are connected to the same WiFi network.

If your both your Cellular Carrier and your calling plan support WiFi Calling, your iPad may be able make and receive calls independently of the iPhone - even when the iPhone is switched off, elsewhere, or using a different WiFi network. Not all carriers support full WiFi calling.

The settings that you need to change/verify on your iPhone and iPad are:

On your iPhone

Settings > Mobile Data >WiFi Calling > 

• WiFi Calling on This iPhone - set to ON
• Add WiFi Calling for Other Devices - set to ON

Settings > Mobile Data > Calls on Other Devices > 

• Allow Calls on Other Devices - set to ON
• [Allow Calls On] - explicitly enable your new iPad from the list of devices.

On your iPad

Settings > FaceTime > Calls from iPhone > Calls from iPhone - set to ON

Additional information about Continuity and the relevant Cellular services can be found here:

Continuity

Work across all of your devices seamlessly – Apple Support

SMS/MMS Messaging

Forward SMS/MMS text messages from your iPhone to your Mac or iPad - Apple Support

WiFi Calling

Make a call with Wi-Fi Calling – Apple Support

Making and receiving calls on iPad

Make and receive phone calls on iPad - Apple Support

When correctly configured, the iPad FaceTime App provides access to Cellular Voice Calling features - whilst Messages supports SMS/MMS messaging services.

iPad can have lots of messaging and audio apps including FaceTime installed, but iPad cannot have the Apple Phone app.

Some other app is missing?

Look in your account details previous purchases in the App Store app, as one possibility. See if the app is listed there. Open it, or redownload it from there.

Turn off Focus or Screen Time or Parental Controls, and check for the favorite app again, as another possibility.