Is it permissible not to go to congregational prayer in the mosque because the shop owner is busy with a customer using a phone?

The most preponderant opinion is that congregational prayer is an individual obligation (fard ayn), based on the Prophet, peace and blessings be upon him, saying to the blind man: "Do you hear the call?" He said: "Yes." He said: "Then respond, for I find no concession for you." So, as long as you hear the call, congregational prayer is obligatory for you and is not waived except for a valid excuse. What you mentioned is not an excuse; rather, you can place a sign on the shop indicating closure before the prayer, and advise customers to first attend the congregational prayer. This is unless the call is for a necessity that permits missing the congregation.