Yes, it is possible to restrict all usage on her phone to only allow her make and receive calls from the trusted numbers only. Within is Usage Controls, you have the option to set up Time Restrictions. During Time Restrictions, the controlled line cannot make or receive calls from anyone other than a Trusted Number and 911. Any calls in progress at the start of a Time Restriction will be disconnected when the restricted time begins. I hope this information is helpful. Please let me know if you require further assistance.
Some phones also have a security setting on the phone itself to limit calls and/or texts to contacts only. Check into that and see if it is an option for her phone.
Most phones I've seen allow you to restrict calls to contacts only. Texts, on the other hand, are usually all or nothing, which means you might not be able to send texts to her phone. Also, keep in mind that the security code on a phone is relatively easy to reset, which will allow her to change the blocks. Plus, even if you block the calls from the phone, the callers can still leave voice mail messages.
The usage controls work great. What I did was set up a time restriction that's 24x7. Then, I put in the numbers we want our son to be able to contact in the trusted numbers list. This allows phone calls and texts from the numbers we want, and callers not on the list can't even leave a voice mail. It's $5 a month, but I think it's worth it.