The purpose of the Call Manager is to replace the conventional PBX. I am assuming you mean analog when you say conventional phones, which are supported for limited use through FXS ports on a voice enabled router.
Cisco's recommendation for the number of phones supported by a single Call Manager is not as cut and dry as you might think. In a lot of the documentation that you will read, Cisco throws around the figure 2500. While you might inclined to think this means phones, in actuality it means "Device Units" with each device that registers with a Call Manager carrying a different weight. For instance an IP phone carries a weight of 1. Doing the math, 2500 phones multiplied by 1 device unit per phone equals 2500 device units. Or Cisco's claim that a single Call Manager can handle 2500 phones. Soft phones on the other hand carries a weight of about 25 I think, and a DSP carries a weigh of 3. So any combination of the above must not exceed 2500. Mind you 2500 is not a hard line, but rather Cisco's recommendation. Performance will begin to degrade the more devices you add that exceed the 2500 device unit limit. Hence the use of clustering.
The majority of our implementations has been to replace the PBX. We usually start with one CM and if redundancy is required or additional nodes is required then we add CM's.
While a conventional POTS line is not required we recommend at least one POTS line in case of a catastophic failure to be used for emergency cases. With the prevalence of cell phones, many customers do not install a POTS line.
A single call manager (MCS 7835) can support 2500 voice "unit". Devices are using defined number of units on a call manager. For example, an IP phone use 1 unit (which mean you can support 2500 phones) but a soft phone use 20 (along with H323 gateway and other stuff).
So to support 2000 phone, I would think a tandem call manager (primary and secondary for redundancy) would do the job nicely.
You also can add servers in a cluster at a later date, to support additionnal phones / users.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.