Paul, call handling itself with VoIP is independent of the ATA or softphone(s) attached. If more than one person calls into the same line, you can configure VOIP.ms (and most other open standards VoIP providers) on how you want handling of any further inbound callers, and can either be handled as something as simple as call waiting, straight to voicemail, ringing multiple subaccount lines or phone destinations as the call comes in (handy for forwarding to cellphones when out of the house!), or shunted off into any number of IVR labyrinths. It can be as simple or complex as you want to make it, but it's (mostly) handled on the VOIP.ms end. That said, some multi-line ATAs have additional complimentary (and even potentially duplicated at times) call handling settings on how multiple incoming calls between separate accounts can be handled as well, either ringing through as call waiting, forwarded to another line by the ATA itself, or shunted back to VOIP.ms for voicemail or call forwarding by registering the extension as busy or offline, etc.
It's also worth pointing out that VOIP.ms lets you set up as many SIP sub-accounts as you like on the main account (think of it as creating virtual line extensions), and because you can handle multiple DIDs on the account, you can route each of those numbers to ring into any specific or all line extensions... but you can even treat those line extensions as separate SIP sub-accounts. You can even assign any outbound Caller ID information you like (including spoofing other phone numbers you own, say, your cell phone numbers) to those specific lines for outbound calls.
This isn't to say that you can't influence and impact phone behavior and call handling with the equipment. Now hang onto your socks, this one's gonna be information dense.
With an ATA like the OBi202, you have support for two physical phone lines out and up to four SIP accounts. With those two lines out, you could either just terminate each line to a separate POTS telephone (corded or cordless), or you could use a two-line combiner kit
(2x RJ11 to 1x RJ14 - never found these on Amazon) in combination with POTS telephones that support two physical call lines, if it's needed and the two line handset doesn't have separate jacks for each line. The only problem is, if you haven't already made the investment in two line handsets or have a source for cheap used on the equipment, the price of new
two line handsets (especially if wireless is desired) can be just as costly (if not more so) and not as flexible (in some applications, but less so in others) as some of the entry-level Grandstream IP telephones. The OBi202 also treats multiple SIP accounts configured on the same physical line with a second call in on a separate account as being handled like traditional call waiting on the phone. You can then prefix the number you dial out on any phone connected to the OBi202 with **1, **2, **3, or **4 to select a specific SIP account (or line if you prefer) for outbound calling to either route through a cheaper VoIP termination provider (email me on a couple of those, I've got some newer options that I haven't added to the guide yet that's worth looking into, but not ready for inclusion) or select a different SIP sub-account to change your outbound Caller ID.
Not to say this couldn't also be done using various internal extensions coupled with DISA
on a VOIP.ms account, but the dial pattern to switch outbound CID credentials before dialing the number you want to connect to becomes considerably
longer than just **x due to the necessity of a four digit security PIN number, but you could also theoretically use a single digit speed dial number on the telephone itself to program in and simplify dialing the extension and PIN, but it has to be a speed dial number that isn't conflicting with your own ATA's speed dial system (if it has it and is configured). That said, you should keep this feature in mind as it can also permit you to return calls from your cell phone to business customers and have them see your business CID number instead.
Getting back to the Grandstream IP telephones and desiring multi-line phones, you have desk phones like the GS-GXP1400 which can handle two SIP accounts as line one and two effectively ($40), or there's cordless handset models like the GS-DP715 (starting at $65 for one handset plus $40 per additional handset all the way to $190 for a kit of five, it's max supported extensions/handsets) where each handset is treated and configured as a specific line extension with its own SIP account credentials, but you'll need to bust out the labelmaker to keep from confusing the phones. Of course, going this route (or any situation where you might have more than two inbound SIP lines at home) and you don't want to run your own Asterisk PBX server, you're going to start running into potential timeout and collision issues on the UDP ports between accounts (especially between the same servers and internal network addresses), but this can be addressed with a little NAT magic and port routing with each extension
if you got decent networking chops (enter the brilliance of using a router that runs DD-WRT, for example - but NAT solutions with VoIP can potentially get messy
, and tomes have been written on the subject). This said, most VoIP providers can use UDP port 5060 or 5080. This can be handy to remember for configuring, say, two inbound
VOIP.ms accounts to your house and phone equipment, thus avoiding any need for a PBX and leverage VOIP.ms's configuration and routing features instead. Just specify a different port for each account. Of course, you lose the ability to call out via different accounts to change outbound Caller ID with these solutions as you're restricted to only two accounts without additional network fiddling.
Like I said, configurations can be as simple to as complex as you want, but what you're after is going to involve a bit
of necessary complexity. I do think it might be worth the effort to potentially learn how to NAT at least four to five extensions, but I suspect the easiest hardware solution setup for your needs will probably be the OBi202 paired with something like the VTech DS6151 two line cordless system paired with a multi-extensioned VOIP.ms account. That'll give you two physical lines that can be used at the same time that'll both ring the same multiple cordless handsets throughout the house but differentiate which incoming line is ringing, have the option for a second (or third) incoming number on either line differentiated by ring profile type (say two short instead of one long), and you can call out with up to two more accounts for either the sake of lower cost termination and/or to switch Caller ID credentials (great for either just having and using a home phone number or fooling people into thinking you're calling from your cell phone when you aren't). It'll also probably be the easiest to configure and provision from a physical hardware standpoint. Be mindful though, the Obihai and VOIP.ms have some overlapping call handling features (not to mention the Obihai having some potentially overlapping NAT features with a router), so be sure to choose and remember which side you want certain features to be handled for routing and failover.
Any specifics you want on setup or need clarification on, you know how to reach me. ;)