It is generally considered unprofessional to answer your phone during a client meeting. When you are in a meeting with a client, they deserve your full attention, and interrupting the meeting to take a phone call can be seen as disrespectful and distracting.
If you are expecting an urgent call that you absolutely must take, it's best to inform your client at the beginning of the meeting. Apologize in advance and let them know that you may need to briefly step out to take an important call. However, make every effort to keep the interruption as short as possible and return to the meeting promptly.
In most cases, it's best to silence or put your phone on vibrate mode before the meeting starts. If there's an urgent matter that requires your immediate attention, you can check your phone discreetly during a natural break or after the meeting.
Remember, your clients' time is valuable, and providing them with your undivided attention during a meeting helps to build trust and demonstrate professionalism.