Here is some info that mark snow posted regarding using 3725's on the lab. I know a lot of people look at these as way to get things working, but there are some IOS changes in the CCNP Voice using CCM8 in the labs that may take away some capability.
Voice lab hardware - PVDM vs PVDM2, E1 vs T1, and ISR vs cheaper routers - IEOC - Internetwork Expert's Online Community
If you are serious about studying for Voice, don't even think of anything short of the ISRs (28xx series), and I'll present many good reasons why, to follow.
1) With ISRs you don'l need NM-HDV's or NM-HDV2's (at least so far as the certification studies go). For the most part, these are simply a means of getting PVDM or PVDM2s into the system. In ISRs, you can put these PVDMs directly onto the motherboard of the router, and put your VWICs into the router/motherboard's HWIC/VWIC slots, reducing the need for spending money on the NM-HDVs.
2)
Even the 3725 doesn't support 12.4(20)T - a vital part of the CCIE Voice Lab. The CCNA and CCNP Voice labs have already surpassed that IOS, and are on to 15.x versions of IOS.
3) Even if only studying for the the CCIE Voice, it is widely believed (based on historical trending evidence) that the CCIE Voice lab will announce an upgrade to the exam in either Dec 2011 or Jan 2012, that will go into effect (begin testing on the new version) around July 2012. This means that in the (albeit possibly unlikely) event that your studying goes on for more than 15 months (which is roughly speaking not too far off from the average studying time of somewhere between 12-18 months), before passing the CCIE Voice lab, you will most definitely need to upgrade your routers to a minimum of 15.1(2)T, but likely beyond that IOS rev. Now, while it is impossible to predict if you may need to go to an even higher IOS version --one that possibly, though I think very unlikely, goes beyond support of the ISR-- I believe you have a far greater chance of being prepared with hardware that already supports the newer 15.x IOS rev.
4) While initial CAPEX cost is higher for an ISR, and while of course the newer ISR-G2s have already been out for some time, it is safe to assume that resale values for an ISR will most certainly be higher than those of a 36xx/37xx series device.
To answer your question about CME versions: It matters a great deal. If you have an older IOS that only supports, say IOS 12.4(15)T (CME Version 4.1), you are missing out on a
lot of features that the newer IOS 12.4(20)T (CME Version 7.0) supports - not to mention later IOS/CME versions used in the current CCNA/CCNP Voice exams (CME 8.0), as well as future potential CCIE Voice versions in 15 months or so (CME 8 or 9.0 likely by then) - and not being able to those CME versions can mean a lot of points left on the table during your exams.
To answer your question about T1 vs E1: Here I will give you a break on cost. At an OSI Layer 1 - T1 and E1 are practically identical. They both use wires 1,2,4 & 5 from a TIA/EIA T568A or T568B structured cabling perspective, and can both use crossover cables to achieve a 'virtual' PSTN using another separate 28xx ISR router (or older - here for your PSTN router you could practically use a 3640/3725 router without much problem).
From a L2/L3 CCS (Common Channel Signaling) perspective (e.g. T1 & E1 ISDN PRI), there isn't much difference at all. In fact, since the PRI L3 signaling of both is identical, the only difference lies in the PRI L2 signaling portion - namely that T1 uses has 23 B (Bearer) Channels, and uses Channel 24 for it D (Data) Channel, while an E1 has 30 B-Channels and 1 D-Channel, which it uses Channel 15 for (both non-canonically speaking, of course).
The only practical differences, as it relates to studying, come when we examine the differences between the L2 usage of T1 vs E1 CAS (Channel Associated Signaling - a.k.a. Robbed Bit Signaling), and then the differences are quite vast. The good news is, while T1 CAS (e.g. E&M Signaling) & E1 CAS (e.g. R2 Signaling)
can technically be tested, it is much more unlikely to occur, due to a number of other stated objectives in testing by the proctors.
So my short answer is - if you only buy T1's for your home lab voice study, you should be fine. And if feel you need practice on E1's, rent a few sessions from GradedLabs to get that E1 practice time in.
On the question of PVDMs vs PVDM2s (vs PVDM3s) SIMM DSP cards: You need PVDM2s or PVDM3s. PVDM2s are good enough for now.
One other thing: About purchasing VWIC or VWIC2s: Here I will give you another break on CAPEX expenditures. While there are a number of onboard physical differences between the Gen1 and Gen2 versions of the VWIC cards, the configuration portion isn't too great that I think you'll notice too much difference. While the current CCIE Voice lab does use VWIC2s, and short of the ability to configure extended echo-cancellers, there is one other huge difference, but something that can simply be studied and
remembered upon entering the lab exam, and it is namely this: Since the VWIC2s can be configured as
either T1s
or E1s (and the router can't read your mind as to which you might wish to configure them as, prior to you doing so), the 'controller' interface to the VWIC2 doesn't show up in running-config until you enter the global command of 'card-type' to define the card as whichever you want - T1 or E1. Other than that one simple command, which again I believe can simply be studied and
remembered upon entering the lab exam, there isn't much other difference that might 'get' you come exam taking time.
However, all of that can be expensive, not the least of which is this note: You
need to have 7961 IP phones at a
minimum. So with all that CAPEX expenditure in mind, why not just rent Voice Rack sessions from INE & GradedLabs? You don't need to buy anything (except rack tokens), you
don't even need a VPN connection any longer (something we just announced 2 days ago), you don't need phones as
we have them all attached to our racks with a free, web browser-based remote phone control client, and you
can rent them for around $15 per 6 hour (5.5 hrs usable) session timeslot.
Given that the average rack-study time to achieve the CCIE Voice (if that's what you are going for at the moment, if not but you do plan to at some point) is estimated at 650 hours, and our 4 daily 6-hour rack sessions contain 5.5 usable hours of rack (30 mins is reserved for inter-session rack reset and both physical & logical connectivity testing), if you purchased 120 sessions for a total of 660 hours of total rack time, and you purchased those rack session tokens in small quantities as you went along in your studies, you would only spend $1,800 USD. If you bought those all up front in bulk, you would only spend $1,440 USD.
I'm not trying to sell you on purchasing our racks, only to say that cost-wise, they far outweigh the initial outlay of CAPEX by you for the hardware, it's already all setup for you, and you can access it from anywhere - no VPN needed. Not to mention there is no risk to you for later trying to have to resell the hardware.
Also one last thing about that, is that when you purchase a bulk set of rack tokens ($1 per token with 15 needed for any one Voice Rack session), you can schedule rack time on our website and cancel at any time, and the tokens used from the cancelled session simply go back into your account's 'token bank' - and there is no need to fuss with a credit card charge and credit every time you book and then cancel.
Anyway, renting may not be right for you for any number of reasons and I certainly can respect and appreciate that - just trying to give you some cost effective alternatives.
Anyway you decide to go, I hope that I've given you some useful information.
