You don't have a choice. Mobile Alert specifically uses At&t. You are not required to change your cell phones to At&t to use these alert systems, it's a stand alone service. They are not compatible with Verizon.
caregiver70, making sure you are aware of your options to purchase a Medical alert system is important. Verizon no longer sells the SureResponce Medical alert system. As @mama23dogs has mentioned you will have the available option to keep your mobile services With Verizon and purchase a Medical Alert system from a separate provider on a different account.