> The strategy does not scale well to non-directed
> pickup. Are you suggesting to register for every phone
> wishing to be pickup capable? Are you are assuming
> a large memory capacity on each phone to monitor
> so many call legs? Forking proxies have
> trouble with PSTN gateways. The REGISTER
> responses would grow very large with big
> pickup groups, not fitting in unfragmented UDP.
> The strategy totally breaks for redirect
> servers or proxies which don't support
> parallel forking.
>
> Other options include ....
> 1) passing all calls through a B2BUA ahead of time.
> Call it a PBX.
> 2) using a remote control protocol to transfer
> the call. The command would
> probably come from a centralized server
> which the phones trust. See
> http://phonectl.sfour.com/
When the client supports REGISTER requests, the Call Pickup
can be done with a REGISTER request directly to the SIP
client that is ringing. What is wrong with the following
scenario:
Client A calls client B. RP does Call Pickup.
A -> B INVITE
B -> A 180 Ringing
RP-> B REGISTER
B -> RP 200 OK
B -> A 302 Moved Temporary
A -> B ACK
A -> RP INVITE
RP-> A 200 OK
A -> RP ACK
With an expire time of zero seconds, the REGISTER request
would not actually register anything, just cancel a possible
registration. However, the client B is allowed to redirect
the current call according to the information in the
REGISTER request.
In a call-center setting, client B could be a SIP server
with queueing capabilities, where each REGISTER request
would redirect the first call in the queue.
Richard Verhoeven.
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