Hi, and welcome to the world of VoIP!
Thanks for doing so much research first, as it makes the discussion more productive.
First, regarding your comment about connecting a VoIP phone to your employer's network: it sounds like you a "work from home" or "telecommuter" employee? If so, and you work for a larger enterprise, they may be using an enterprise-class VoIP system from Cisco, Avaya, or Microsoft. If so, they may allow remote connections to their network. You can always ask.
Let's assume that it's not an option, for the remainder of the discussion. You want a home VoIP phone system. It needs to include cordless features. It needs to support multiple different VoIP service providers. The Panasonic model you mentioned is obsolete. Their new model is the KX-TGP600. It looks like a great system. Panasonic has a very good track record in VoIP, and is the leader in DECT phone systems. I looked at the phone's spec sheet, and it doesn't mention how many (if more than one) service provider it can register-to at the same time. This has been a deal-killer for some of the earlier DECT/SIP systems, in that they only worked with one service provider (only one configuration stored for a SIP userID/password/SIP server). Feel free to investigate and let us know what you learn.
Let's discuss OBi-based solutions. OBi makes outstanding VoIP desk phones, with great audio quality and support for carriers that offer HD-Voice (wideband audio CODEC) connections. They don't have companion DECT cordless handset options. However, you can use the OBi 1032 or 1062 desk phones with a DECT digital cordless headset from Jabra/GN Netcom, or from Plantronics. I have a 1032 connected to a Jabra PRO 9470 headset. This is a business-class, high-quality DECT headset, with outstanding range, long talk-time, very high sound quality, and compatibility with the Electronic Hook Switch (EHS) function on the 1032 and 1062 phones. This means that you can wander around the house and answer calls by clicking a button on the headset.
See:
http://www.hellodirect.com/hellodirect/Shop?DSP=30102&PCR=1:1:5:15:150:1040&IID=13577&itemskuid=13577Another OBi solution is to buy a OBi 200 or 202 ATA, and connect any conventional RJ-11 attached DECT cordless phone system with multiple handsets. Again, the Panasonic models are excellent choices.
If you're mostly dialing into conference calls, either solution would be fine.
The OBi IP phones support up to six different VoIP service provider "slots" and the OBi 200/202 models support up to four service providers. One or more of them can be Google Voice. OOMA is a closed, proprietary system, which doesn't give out its server credentials, so it can only be used with their own adapters.
With regard to conference calling: Google Voice doesn't care how many minutes per month you are on calls. However, they do limit each individual call to three hours. At that point, you'd need to call back in, and or look for a new job that doesn't make you stay on calls that long
You can contact Callcentric and
voip.ms to find out how they deal with toll-free calling. It likely doesn't matter to them either, since they aren't paying for the calls; in fact, AT&T is paying them to connect the calls, as with any toll-free number. A
voip.ms user probably knows the the answer as to their policy and whether or not they charge the caller for calling TFNs. Callcentric doesn't charge the user for calls to TFNs.
I would not try to mess with a complex interconnection of your OOMA service to the OBi. The OBiLINE adapters don't work very well, and the OBi 110 is not as flexible or "future proof" as the 200 series. Just get the SIP VoIP service(s) set up, try them for a while, and then dump the OOMA service. In the interim, just hook up a separate telephone to the OOMA box and use it for the OOMA calls.