I assume then that your release conditions prohibit you from communicating with their mom or from being around the residence.
If you are not prohibited from communicating with your children, then there is nothing to prevent you from sending them a package including phones, to the home. I would not do so if they are rather young however. Also, if you were to do this, you would make sure that there is nothing about the package or any accompanying letter that could be construed as being communication with your ex, either directly or indirectly, otherwise you'll get breached.
Alternatively, you could always ask a third party to provide the phones to the kids so that they can contact you. This would probably be safest.
Ultimately, you do not want anything to be misconstrued as any form of communication either direct or indirect with your ex, otherwise you'll be charged and be back in custody.
Also, if you are involved with the Family Courts, you can seek to have this facilitated through the Family Court process.
Please let me know if you require anything further.
Also, please take a moment to leave me a positive rating.