I am working on figuring this out. I have gotten half of the way:
The following is meant to be a concatenation of the phone and plus icons:
.fa-phone-plus:before {
content: "\f095";
}
.fa-phone-plus:after {
content: "\f067";
position: absolute;
font-size: 0.5em
}

It's not perfect as it starts to cut off the phone on the left. But at the moment it does what I need it to do.