Example: I would like to use the WhatsApp or Telegram icon that exist in Fontawesome

Depends on what element you want to apply it to, the theme font icons are a spin of Fontawesome (old version) and other custom icons so it would be harder to achieve.
I can help you with a custom icon though, where would you like to use those?

.boc_list_item .icon:before {
    content: "";
}
.boc_list_item .icon {
    background: url(https://i.pinimg.com/originals/93/a8/80/93a88060254b54b01ab3ee8a5f1dbaba.png) no-repeat;
    width: 26px!important;
    height: 26px!important;
    display: inline-block;
    background-size: cover;
}

Few notes:
1. Obviously the icon image I used is on some CDN out there, you might want to upload your own image locally and set your own url.
2. Currently this will apply to all list items on the current page, if you want to use other icons you can apply this only for a single icon (override only that icon) so:

.boc_list_item .icon.icon-album:before {
    content: "";
}
.boc_list_item .icon.icon-album {
    background: url(https://i.pinimg.com/originals/93/a8/80/93a88060254b54b01ab3ee8a5f1dbaba.png) no-repeat;
    width: 26px!important;
    height: 26px!important;
    display: inline-block;
    background-size: cover;
}

Here I am overriding the Album icon (first one in the list), so picking that icon will use your custom icon. Hope that helps!