WRTU54G-TM router with 3rd party firmware; Can custom firmware include stock binary portions?

WRTU54G-TM router with 3rd party firmware; Can custom firmware include stock binary portions?

I've been doing a lot of reading online about the Linksys WRTU54G-TM router model that I now own. It seems getting a custom firmware onto it is not a problem. But no one is talking about retaining the Voip features (yet). So far they're all disappointed that it's not a SIP machine and used GSM over IPSec.