As with the computer, I will leave the choice of a phone up to them. If they wish to get a cheap “free” phone from a carrier, that is their choice. If they wish to invest in something that is more important to them, then that is their choice as well. They do not have to acquire a cellular phone. Obviously, not having a cellular phone will mean they won’t be able to use IM services such as RCS or iMessage. They can do something like acquiring a Networked DECT? phone, or a wifi? phone, and let it be at that. The choice is theirs, but they will use the cash gift to acquire the phone(s) they wish to have.
In addition to a phone, they must acquire a voicemail number. I will encourage them to have a website beyond Facebook or Instagram. With that said, they will likely need their own domain, and their own email address. The voicemail number is so the foster child does not get harassed a dozen times a day on Web Design services when Word Press (what made this site) can meet their needs.