Can you communicate from your apple watch to your apple phone?

No, and maybe from the iPhone to the Apple Watch (MAYBE).

The iPhone and Apple Watch share the same phone number.

Anyone that calls the iPhone phone number will also ring the Apple Watch.

Most carriers (not all; but most) assign a billing number to the Apple Watch which looks just like a phone number.

Some carriers allow you to actually call the billing number (but not all carriers). When that happens just the Apple Watch rings.

But the Apple Watch could not call the iPhone, as to the carrier that would be calling your self, and your line if busy because you are dialing a number.

Again, NOT ALL carriers even allow calling the Apple Watch billing number.

Also like lkrupp said, it must be a cellular Apple Watch that has paired to an iPhone and is sharing the iPhone's number.

Now there is such a thing as an Apple Watch Family Plan, that allows a parent to control a dependent child's Apple Watch. There are a ton of restrictions and features that do not work, because the dependent child's Apple Watch does not have an iPhone to do the heavy lifting. In this configuration the Apple Watch does have its own phone number, and the Apple Watch can call the controlling parent's iPhone, and the parent can call the dependent child's Apple Watch.

But this is not the same as your question, which is just letting a friend take your Apple Watch.

First, you should ONLY be paying the equivalent of $10/month U.S. for the Apple Watch to share the iPhone phone number. In the U.S. some carriers will even do the sharing for $5/month (then again, that was T-Mobile, and they just announced some rate hikes, so I'm not sure if it is still $5/month).

If you are paying a lot more than $10/month U.S., then I would look for a different carrier for your iPhone and Apple Watch.

The cellular carrier may assign a billing number to the Apple Watch, but for the most part the cellular carrier treats both devices as having the same phone number. I already mentioned that some carriers may let you call the Apple Watch billing number, but that is not universal, and it is not the direction you want to call, as per your questions.

And yes your friend can call on your phone number and appear to be you in text messages to all her friends. She just cannot call the phone number she is using to make phone calls. That has never been a telephone feature, and I go back to the days when you picked up the telephone, and you told the operator you wanted to call some number (there was no keypad, nor rotary dial on the telephone), or in a small town, you could just ask the operator to find your mother, and they would know which house to call.

Also, giving your Apple Watch to someone to use, is NOT how the Apple Watch was designed.

But if you really want to do that, then switch the Apple Watch to the Apple Watch Family Plan, and get a different cellular plan for your Apple Watch.

Apple Watch Family Plan

Set up Apple Watch for a family member - Apple Support

Set up a family member’s Apple Watch - Apple Support

Some limitations: Cannot receive SMS/MMS messages, as that depends on a fully paired iPhone. Messaging is pretty much limited to Apple's Messages, unless the messaging service provides an Apple Watch app. Alarms, alerts, notifications on the iPhone are not sent to the Apple Watch. Apple Pay via a credit card is not available. Also not available: Medications, respiratory rate, irregular heart rhythm notifications, ECG, AFib History, Cycle Tracking, Sleep, Wrist Temperature, Blood Oxygen, Walking Steadiness, Audiobooks, Camera Remote, News, Shortcuts, and the double tap gesture.

If $10 U.S. added to your iPhone is too expensive for the Apple Watch service, that will not allow the Apple Watch to call your iPhone, maybe the Apple Watch is not the correct smart watch for you. There is nothing wrong with using a non-Apple product.

First and foremost the Watch in question would have be a model that has a cellular modem and a cellular plan activated for it. Otherwise the watch can’t call anything without being close to its paired iPhone. Second, the iPhone number and Watch number, while different, are treated as the same number by your cellular carrier. If someone calls your iPhone number it will also ring yourWatch. When your watch is separated from your iPhone your watch can only rely on its cellular connection (if it has a cellular modem in it.) It cant use Bluetooth or anything else to connect your iPhone.

How to pair your Apple Watch with a new iPhone - Apple Support

Set up your Apple Watch - Apple Support