is a lying to your supervisor a serious misconduct?

phone are not allowed inside our company because its a bank. but that morning i just forgot to leave my cellphone in my locker. the security guard caught it bulging from my jean's pocket.
i thought my supervisor could help me with the situation so i approached him and asked him if he could help me. then he asked if what's in my pocket is really a cellphone.
i said no, it's something else. i was meaning to ask him if he could help me make the guard realize that I was not really using the phone. i just forgot to leave it in my locker. he didn't look like he would help me so later on i still decided to surrender it to the security guard.
they are now accusing me of serious misconduct and willful disobedience which could result to termination.

you have three offense(s) actually:

1. bringing in cellphone
2. lying to your superior
3. if you're in Makati, i know it's illegal (there's actually a Republic of Makati Law) to use phone inside banks (like phones on gas stations)

now let's looks at your defense: the reason of "i forgot" is not valid, and then instead of telling it to your superior, you elected to "not tell" intentionally. and lastly, it's a city government offense. now, here's what our labor code has against it that the arbiter may take a look at if they (bank) opt to terminate you and you filed a case with NLRC:

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Causes for Termination by the Employer

1. Serious misconduct;
2. Willful disobedience of employer’s lawful orders connected with work;
3. Gross and habitual neglect of duty;
4. Fraud or breach of trust;
5. Commission of a crime or offense against the employer, employer’s family or representative; and
6. Other analogous causes.

Due process is a must.