Whose contact information should be written in Paragraph 21 of TREC contracts to receive notices?

There isn’t one correct answer to that question. However, you could consider using the buyers’ and sellers’ contact information.

Why? Because time is of the essence in almost all of the notice provisions in TREC contracts, meaning they require action before a strict deadline. Having an agent as the point of contact to receive notices for his or her client could create delays that may result in the party losing a time-sensitive option or right provided in the contract, such as the buyer’s requirement to waive their contingency after the seller accepts a backup contract under the Addendum For Sale Of Other Property By Buyer (TAR 1908, TREC 10-6).

You might be reluctant to use the buyers’ and sellers’ contact info because you think direct contact with the other party is forbidden. However, as long as you’re using the contact information to provide notice to the other party, you’re not crossing the boundary of soliciting another agent’s client.