JavaScript error onload in Account form

Hi,

Our contractor wrote a javascript function to format the phone number field in the Account form. I tried to reuse the same function in another Account form, which I thought worked fine.

But, now I am randomly getting Script error on some accounts, which doesn't happen every time - See screenshot below.

I am not sure what the issue and I am new to JavaScript as well.

Any help is much appreciated.

Thanks,

Praveen.