Absolutely!
There's nothing to stop you from using VoIPBuster on your WM5 device. Simply download and install the free WM5 beta version of SJPhone
here, then follow the instructions
here. Note: VoIPBuster is one of the
Finarea SA group of companies and hence the settings given in the previous link are almost identical to the ones needed for VoIPBuster. The only difference being the SIP server and your user credentials.
Cheers