[SIPForum-discussion] Question Regarding "One Number, Multiple Phones"

Seelaender, Roger Roger.Seelaender at elnk.com
Mon Aug 10 13:33:56 UTC 2015


This is quite common in SIP with a possible advantage of Hold/Restore behavior that you do not get in PSTN, unless you have a key system there. Just allow multiple registrations on your SIP server to the same user name or DN and make sure that your Proxy identifies each registration with a unique tag. Incoming calls will ring all phones and you can answer on any. 

As to the Hold/Restore behavior I mention, that depends on a SIP feature called Line State Monitoring. If enabled, the phones will Subscribe to themselves at boot up and usually refresh that behavior about once an hour. When enabled and you answer the call and put it on hold, you will see every set on that number reflecting the held call and you can retrieve the call on any of them. If you do not enable that feature, the call will be held only on the set that put it on hold. All the other sets are still available on their line appearance for additional incoming and outgoing calls. To some folks this is an advantage over PSTN behavior.

Hope this helps.

Roger Seelaender
VoIP Architect 

-----Original Message-----
From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of SMITH, BILLY R GS-11 USAF AETC 81 TRSS/TSUS
Sent: Thursday, July 23, 2015 8:45 AM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] Question Regarding "One Number,Multiple Phones"


I have a question regarding "One Number, Multiple Phones".  In a standard PSTN household, a family may have many phones in different rooms.  When there is an incoming call, all phones ring and any one can answer it.  Most importantly, at any time, any one of the other phones can pick up on the call.  Is this possible using SIP?  I have read the RFC, but being a VoIP novice, I am unable to see where this is addressed.  From what I understand, some vendors (i.e. BroadSoft) have addressed this need in a proprietary manner, but it has not been addressed as a general feature of SIP.

This is not simply an academic issue for me.  We are in the process of replacing a meridian mini switch with a VoIP softswitch.  This is for a classroom configuration where students are in a simulated "Control Center" and they interact with the Instructor who acts as numerous outside agencies.  There is no connectivity outside the classroom.  In the real world Control Centers we are simulating, there is the ability to have the same number on a multi-line phone at various working positions.  At any time, students working in these positions need to be able to answer and/or join in to an existing call.  

I understand there are ways within SIP to set up "conferences" that allow you join conversations using a dial-in/password sort of thing, but I need it to act just like an ordinary PSTN phone.

The current VoIP soft switch we are using provides most of the functionality I am looking for.  It allows up to three phones to register with the same number.  Incoming calls ring on all three handsets and any of them can answer the call.  BUT, here is the problem.  Once one of them answers the call, all others are blocked from joining the conversation.  Do you know of any way for me to accomplish this?

I appreciate any light you can shed on this topic,

Billy Smith

	Billy R. Smith, CIV, DAF
	Programmer Analyst, 81 TRSS/TSUS
	DSN: 591-8448  Comm: (228) 376-8448

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