Probably you addressed this query to a portal which is predominately for Indian Taxation.
A quick look at the treaty between Philippines and USA gives the impression that your tax liability depends upon the interpretation whether you have permanent establishment in USA or not, as you flew one of your trainers to the USA. In my view, it may not be possible to consider the temporary stay of the trainers as having permanent establishment, unless there are other facts, which may prove it otherwise.
As the source of income is from USA, you will have tax liability in USA in respect of the service rendered by you but the credit for such tax liability can be used for setting off your tax liability in Phillipines, as per the applicable tax laws.
As regards your specific queries,
a) I think the treaty has nothing to lower the rate of 30%
b) If you have permanent establishment in US, you may lower the tax liability by claiming expenses incurred on such service.