This is a good an interesting question, and I will give a detailed explanation trying to clarify some aspects for shared line implementation:
First of all I checked the code, there is no max limit SET in CCM. But 200 is tested limit.
These have been tested with all phones on the same LAN.
Check this URL
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/6x/netstruc.html#wp1045335
Scroll down to the section titled:
Considerations for Shared Line Appearances
I will mention as an example 100 phones, the main idea here is that those 100 phones will each require about 4 packets (assuming SCCP, and 6.x). That is 400 packets generated quasi-simultaneously by the UCM server (assuming they are all in the same device pool). In case those phones are over the WAN, by default, your WAN interface will be able to accept 64 of those packets (coming in at 100, or 1000Mbps). The time it takes to 400! serialize 1 packet may be more than the time it takes CUCM to generate
So the tail end of the 400 packet train will be dropped. Eventually re-transmitted, yes, but perhaps too late to avoid the perception of some
phones "lagging" in their response to UCM stimuli: they may ring late,not ring at all if another phone picks up rapidly and all ringing is
cancelled, they may even re-register if the re-xmit sequences exceed allowable limits.
Increasing the queue depth is the only weapon. Increasing BW is less effective, unless you can approach the BW that connects CUCM servers
to the WAN router.
So when you use shared lines over the WAN make sure you check above calcualtions and scenarios
Few Shared line restrictions:
1) Do not use shared lines for Cisco Unified Communications Manager
Attendant Console pilot points or hunt group members. The phone that
acts as the attendant console supports shared lines.
2) Do not use shared-line appearances on any Cisco Unified IP Phone that
requires autoanswer capability and do not turn on auto answer for a
shared-line appearance.
3) Do not configure shared-line appearances on the primary lines of the
phones; for example, if two phones have a shared-line appearance, only
one of the phones should have the primary line configured as shared (the
other phone should have the secondary line configured as shared).
Barge, cBarge, and Privacy work with shared lines only.
4) Cisco recommends that you do not configure shared lines for Cisco
Unified IP Phones, H.323 clients, and MGCP POTS phones; likewise, Cisco
recommends that you do not configure shared lines for H.323 clients and
MGCP POTS phones. If you configure the same shared-line appearance for a
H.323 client, a MGCP POTS phone, for example, NetMeeting, and a Cisco
Unified IP Phone, you cannot use the hold/resume feature on the H.323
client or MGCP POTS phone.
5) Cisco recommends that you do not configure shared lines for Cisco
Unified IP Phones 7905, 7912, 7940, and 7960 that are running SIP
because these phones cannot pick up held calls on shared lines nor can
they use the shared-line features Single Button Barge/cBarge, Barge,
cBarge, and Privacy.
6) Cisco Unified IP Phones 7906, 7911, 7941, 7961, 7970, and 7971 that
are running SIP have the capability of supporting multiple lines with
the same directory number in different partitions. However, configuring
and using other SIP phones with multiple lines with the same directory
number is not supported.
HTH
