Call or SMS to inform whether the change of bid opening time is valid.

According to the "Regulations on the Implementation of the Bidding Law": "Article 21 A tenderer may make necessary clarifications or amendments to the pre-qualification documents or bidding documents that have been issued. If clarification or modification may affect the preparation of prequalification application documents or bidding documents, the tenderer shall notify all potential bidders who have obtained prequalification documents or bidding documents in writing at least 3 days before the deadline for submitting prequalification application documents or at least 15 days before the deadline for bidding; If it is less than 3 days or 15 days, the tenderer shall postpone the deadline for submitting pre-qualification application documents or bidding documents accordingly. "

Text messages or phone calls are not in written form, so they are not in compliance with the law.