Based on the information you provided, it's difficult to give a specific answer without more context. However, there are a few possibilities to consider:
Hidden Phone or Device: It's possible that your boyfriend has a hidden device, such as a Bluetooth earpiece or a small, discreet wireless earphone, through which he's communicating with the other person. These devices can be easily concealed in the ear, making them difficult to notice.
In-Ear Headphones: Some modern in-ear headphones have built-in microphones that allow for hands-free phone calls. If your boyfriend is wearing these headphones, he could be talking to someone using his phone without having to hold it in his hand.
Speakerphone: If your boyfriend has his phone on speaker mode and it's placed in a pocket or somewhere nearby, the sound from the call could be loud enough for you to hear both sides of the conversation.
Voice-activated Devices: There are voice-activated devices, such as smart speakers or virtual assistants like Amazon Echo or Google Home, that can initiate and receive calls without the need for a physical phone. These devices can be controlled through voice commands, allowing for hands-free communication.
Sound Amplification: If you have particularly keen hearing or are in close proximity to your boyfriend and the person he's talking to, you might be able to pick up on the conversation even if they are speaking quietly.
It's important to communicate openly and honestly with your boyfriend if you have concerns about his communication habits. Trust and communication are vital in any relationship, and discussing your feelings and observations can help to address any misunderstandings or discomfort.