Am I entitled to a commission for a real estate sale that was made with a former client of my old company after I left the company, knowing that communication took place via my personal SIM card and there is no clause in the contract that prohibits this?

If you leave a real estate company and a former client contacts you to purchase a property, the old company has no right to a commission, as long as the contact regarding the property occurred after you left the company. However, if the contact happened before you left the company and the purchase was made after you joined another company, then the first company is entitled to a percentage of the commission. There is no harm in dealing with clients of the old company.