Phone Link, 3.5mm Plug
The Phone Link allows you to connect between a phone and a communication device. It should not matter if it is an iPad or other voice output device. The sound comes from the 3.5mm earphone jack of the device and sends that output directly into the phone. The person will still need a way to answer or open the phone line, and will need to hold the handset or use a headset to hear the person on the other end of the phone.
An alternative is to use a speakerphone so the person can speak without connecting the Phone Link or holding the receiver. This works well in a quiet room without a lot of noise, but will not offer the same amount of privacy that the Phone Link provides.
More information: https://www.zygo-usa.com/usa/index.php/our-products/augmentative-devices/text-to-speech-devices/accessories/phone-link,-3-5mm-plug-detail