Ignore the post from Azrobert.
Yes, you can accomplish exactly what you want with either Callcentric or
voip.ms. I have been using Callcentric for over a decade, and Lavarock7 has had extensive experience with
voip.ms. Both are solid options, and both will work with OBi IP phones.
After you sign up for service with one of those two providers, you'll need to decide how many inbound telephone numbers you need, and how many extensions you need. Next, you'll configure the extensions, assign each extension a SIP password, and set up your call treatments (inbound call routing), and set up your IVR prompts and actions.
Once that's done, you will use the OBiTALK web portal to add the IP phones and configure their SP (service provider) slots to use the extensions you created previously. To do this, follow the instructions to use the OBiTALK **5 xxxx method to add the new devices to your OBiTALK dashboard:
http://www.obitalk.com/obinet/pg/obhdevYou'll be asked if you want to configure Google Voice - decline that. Then, scroll down to the bottom of the page and click on "OBiTALK compatible service providers". On the next page, select Callcentric or
voip.ms as appropriate, and fill in the SIP credentials on the form. That's the end of the basic configuration. Of course, you will want to label the line keys and whatever settings you wish.