I’m gonna guess (based on some of his other posts) that he’s dealing with an ATA, so his description might not be too far off.
@TechGuyMike, if I may, let me help a little:
BLUF: Don’t seize the trunk - add inexpensive capacity.
In analog telephony, a trunk is a collection of lines. A trunk is in use if 1 line is in use (as @david55 points out), but if the trunk is full (all lines are in use), you need to find a way to force capacity. Where I come from, we call this “Flash” and “Flash Override”, which releases assets to place your call. In fact, “Flash Override” is the analog equivalent of what @dicko suggested. It hangs up everyone’s calls and clears the lines for Emergency Action Messaging. It also prevents “non Flash” traffic from getting onto the switch.
To answer your original question requires us all to agree on the vocabulary we’re using. One way that can help is to differentiate a “SIP Trunk” and an “Analog Trunk” (or a T1, a Primary Rate Interface, or PRI trunk). The reason it’s important is because Analog trunks provide a limited number of lines in hardware, so there’s no flexibility for capacity. When you hit line 24 on a T1, you’re done. That’s there Flash comes in.
With most VOIP providers, the limit is contractual. You get a certain number of outbound simultaneous calls in your “package” and if you exceed that number, you pay extra. In this scenario, you don’t have to worry about the trunk being full, because you just get charged more for the extra lines.
Most VOIP providers also charge extra for 911 support for a trunk. For example, Voip Innovations (now a Sangoma company) charges a couple dollars a month to support 911, and $300 for a rogue 911 call on a non-911 trunk.
IIRC, @TechGuyMike, you’re using an ATA to support 4 analog lines from the phone company. If that’s the case, it’s up to you how you’re going to handle this. You have several choices:
- Dedicate one of your analog lines to Emergency Services and use the available commands in your ATA to seize that line for outbound emergency calls.
- Get an account on an ITSP that supports e911 services (Telnyx and VI are both reasonable choices in my experience) and only use that SIP connection for Emergency Services.
- Move away from the ATA altogether and move to a more flexible (and probably considerably cheaper) SIP primary solution. You can certainly leave one of your analog lines (since they have B+ support) as your emergency line.
So, in the olden days, your question would have been one that we would have had to debate and decide the least disruptive solution, but with modern telephony solutions, the choices are all flexible and inexpensive. Bottom line: add flexible capacity to support your anticipated e911 needs and mitigate the concern you have about your trunk config.