Do i need asterix

Hi,
I have 2 ploycom sound point ip 500h phones one at my home & one at my office. I need to configure them so that they work over internet. is it necessary to have asterix or can it be done with asterix. Planning to use dyndns for hosting and planning to forward the port. but dono which port to fwd any little help would we great.

Thanks.
Kanted