Bluetooth Handset Not Making Calls when iPhone SE in Assistive Access

My mother is disabled and has difficulty getting to her phone (iPhone SE, 3rd Gen) when it’s across her apartment. To help, I connected a cordless phone with multiple handsets to her phone by Bluetooth. The problem is when her phone is in Assistive Access she is unable to make phone calls using the Bluetooth handsets. Phone calls received while her phone is in Assistive Access do ring the handsets and she’s able to pick up using the handsets. I confirmed that she is able to make calls using the handsets when her phone is in Assistive Access. Is there a setting that needs to be changed to allow for her to make calls using the handsets in Assistive Access?