Dialing correct conference code results in error from conference system

I just witnessed my boss dialing into a conference line, entering the correct conference code multiple times, and the conference system saying that the meeting does not exist. He tried entering the correct conference code slower but it still did not work. I asked him to use his mobile phone and he was able to dial in to the conference without any issue. People were on the line waiting for him.

What is causing this issue? Is it the FreePBX/Asterisk system or the Sangoma phone? His phone is S700.

Any thoughts?

Thanks in advance!