when you said that you were given a choice of the date for your suspension which you allegedly did, this means you have agreed on your supervisor's terms. Now, in this circumstance, if you signed a document pertinent to the sanction given to you it would be interpreted as valid. Now if this was done and you are not in agreement and documents will show that you are questioning the legality of you suspension then it would be a diffrent story.
yes, you can consider it illegal because you were made to believe by your supervisor that someone saw you violating i suppose your company policy on using a cellphone while at work and ask you not to respond to the show cause memo given to you.but if you will question the legality with your HR or worse with NLRC, you may be burning your bridge with your employer and this questioning might be given an interpretation prejudicial to you by your employer.but if you think that your rights as an employee was violated, under the law you can take actions to redress a wrong done to you by your employer. just need to be ready to substantiate all your cause of action with evidence to prove your case.