I have a 3725 and was wondering if I could do basic VoIP without Unity or Call Manager? Is it possible to use the router to connect to POTS phone lines but use Cisco IP phones internally? If basic VoIP is possible what features would be available for me to use?
What you're looking for is called Callmanager Express (CME). It allows you to run up to ~100 phones on the 3725 without requiring a full Callmanager. For basic voicemail, you would want either something like the AIM-CUE or the NME-CUE (Cisco Unity Express) which is an AIM module on the motherboard and a NM card, respectively.
CME will run on any ipvoice image, and is configured via the telephony-service, ephone-dn, and ephone configurations.
Here's the administrator's guide:
For POTS lines you would want a VIC2-2FXO or something similar. With CME, the FXO card, and a couple of phones, you could be running a full voice solution with minimal effort. You can take a look at the features available on CME, and nearly all of the standard Callmanager features are available.
The CME runs right on the router. An example setup I'm running at home is a Cisco 7960 IP phone registered to my 3745. I also have my Vonage soft phone account registered to the router allowing me to make calls out via SIP VoIP. I have an FXS module that provides another way to access the phone system. Hope this helps.