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

nttranbao nttranbao at yahoo.com
Mon Aug 10 15:44:07 UTC 2015

IMO there is no exact duplication in SIP world as you expected. To join a existing call, IP Phones need to dial a number first. Maybe you can do this with an ATA device hooking up to multiple analog phones.
If you want to do with SIP phone, then the first option I can think of is using "spying" feature, if your softswitch/PBX supports. With this approach SIP phone needs to dial a specific custom code (i.e. *99) and soft-switch will "hijack" the desired ongoing call. Alternatively you may route incoming call to a conference room, and use a custom code to join it. Again, this will be done in soft-switch side. Finally, some IP phones has "hotline" feature to dial a specific number when going off-hook, so you may want to give it a try.
Best regards,

Bao Nguyen
      From: "SMITH, BILLY R GS-11 USAF AETC 81 TRSS/TSUS" <billy.smith.5 at us.af.mil>
 To: "discussion at sipforum.org" <discussion at sipforum.org> 
 Sent: Thursday, July 23, 2015 9:44 AM
 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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