Locally-implemented PBXs are outdated technology for business telephony. Cloud-based telephony, which is much more versatile, flexible, and able to integrate with other I/T systems, is the current and future solution.
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The system is an old (c. 1997) Altigen
If you look at Altigen's website, you'll see that they have become cloud-based, offering telecom solutions as a service, instead of just selling PBXs.
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I'm not sure what you mean by, "SIP based IP-PBX, or a hybrid one," which sounds pretty technical.
That wasn't very clear wording. Instead, compare your existing telephone service, hardware and telephone equipment setup with one where your only on-premises equipment are individual telephones. Each telephone has an Ethernet connection to your office's LAN. The telephones can even get their DC power over Ethernet, with the appropriate network switch or POE injectors. There is no on-premises PBX. All the inbound call handling logic is configured on a web browser "portal" page, provided by your Internet Telephone Service Provider (ITSP).
In this scenario, you would port your toll-free phone number, and your main inbound telephone number, out of their current carriers, and over to the ITSP and its carrier partner(s). You could then easily configure any sort of hunt group, call handling for the toll free number, and features such as voicemail, outbound caller ID, etc.
These services are much cheaper than your current solution, don't require much on-site technical support, and are very easy to change over time (add or delete extensions, change call handling behavior, etc).
You can select from ITSPs that handle the provisioning (setup) for you, along with "hand-holding" ongoing customer support, or you can select a ITSP that is more DIY and a la carte as far as features go.
For example, Callcentric, PhonePower and
voip.ms are three of the DIY ITSPs, typically with only email-based support, or, you can go with a cloud based provider that is tailored to your needs (for example, if you want/need integration with Salesforce or Office365 or your CRM system, or if you simply want a "make it work and I don't need to know how it works" service). Examples of that are Nextiva, RingCentral, or DialPad (or, Altigen, for that matter).
Google Voice is not suitable for your business needs.
You do not need an OBi 504/508, nor a dozen OBi ATAs. All you need are IP phones, and Obihai makes great ones: the 1022, 1032 or 1062.
Your ITSP can work with you to set up a cloud-based replacement for your existing setup. You'd typically have only two inbound numbers: the current TFN, and the Comcast number. The ITSP then supplies enough "channels" to support whatever size hunt group of extensions you have. Think of a channel as the ability to make or receive one phone call at a time, not as a phone number. You can add as many channels as you need, and you can change your capacity at any time, simply by making the change with the ITSP remotely. Of course, if you wanted each user to have their own, distinct outbound telephone number, that can also be accommodated.
Outbound calls can show either the TFN or the former Comcast number's caller ID, as you wish.