At my company, we have several offices all with good bandwidth currently using VoIP phones through a service provider 8x8. While 8x8 is good, I have been instructed to research bringing our VoIP solution in-house to cut costs.
So, not having any experience with this, I've done some research into DID numbers and Openfire, but nothing really tying everything together.
I'd like to continue using Ubuntu (Xubuntu to be exact) for this solution. Everyone needs to have an actual phone sitting at their desks. Low-bandwidth drivers would be great. If anyone has any advice, I'd greatly appreciate it.

Exactly what kind of service do you need? Are you primarily communicating with external subscribers, or is there a lot of inter-site traffic as well?

If you're currently using an external provider, all VoIP/SIP traffic goes through that provider. An internal SIP gateway (like SER) would keep all internal calls local, which may or may not affect the size of your bills. Also, such a setup would not require a separate, external SIP account for each phone.

If you'd like features such as voicemail (with or without e-mail integration), directories, IVR menus, presence, day/night setting etc. could be of interest, you should consider a software PBX like Asterisk.