How to adequately add a resource group with a single request to the REST API?

Question

RESTful API

Let's assume that our data storage is arranged in the following way:

There was a need to implement a REST API to get/change data in the system.
It is logical to assume the following structure:
GET /users/ - getting a list of users ( ?expand=phones - phones associated with users will be received )
GET /users/{id} - getting data about a specific user ( ?expand=phones - phones associated with a given user )
POST /users/ - adding a user
PUT /users/{id} - editing user data
Same with phones.
To implement communication, we will do the same for user_phone.
And now the question itself:
The REST API involves adding only one resource using the POST method, but what if I need to bind 100 phones to a user? 100 requests in this case is somehow too much, obviously this problem needs to be solved in a different way.
Possible options:
1) Bypassing the recommendations, send an array with these resources to POST /user-phones/, for example: [{"id_user":1,"id_phone":1},{"id_user":1,"id_phone": 2}]. But then it is not clear what response and what statuses to return, how to mark errors, etc.
2) When editing the PUT /user/{id} resource, pass some phones value that will contain an array consisting of phone ids, for example: {"name ":"Alyosha","phones":[1,2,3,4,5]}.
How to do better? Are there adequate solutions to this problem, or should one of these two options be used?