Withholding Tax Query

[smartecosse]

if i bill a company here in Thailand for 100K of work 3 % is "retained" for the withholding tax

however, if i bill the same company for the 100K but to their Singapore office that office obvioulsy does not retain 3%

Should i be declaring these monies billed outside of thailand to someone as they are being paid into a Thai account and should i be paying the 3%??

thanks