Beginner questions about trunks


I run an IT consulting company and I would like to eventually offer VoIP consulting to my clients. I have been studying VoIP (particularly asterisk) for a little while now, but a few things are not perfectly clear.

I just want to clarify what kind of connections I might encounter from the telcos and what equipment will be needed for each. I know that all telcos will offer different options, but I am just looking to be ready for any connection options they may have.

  1. Regular POTS/PSTN phone line.
    This will require a card like the digium TDM410 or the TDP800P with an FXS module. I assume this will just use a regular RJ-11 jack.
    If the customer needs several lines, they would just get several individual POTS lines each requiring a new RJ-11 jack in the card. With this option, do Telco’s generally allow multiple lines with only one number for this?

  2. PRI / T1
    When a customer has several lines, they would get a T1 / PRI connection and the softPBX would have a card like the digium TE420 or TE122.
    I see that the digium cards seem to have and RJ-45 jack, would it be safe to assume that is how the T1’s normally come in?
    With this setup, do customers usually have the option of having several lines with only one number or one number per line or as many numbers as desired?

  3. ISDN
    If a customer only needed 2 lines, would an ISDN be a common connection? Or would they usually just use 2 POTS?
    Is there a digium card for this?

  4. Centrex
    Are these usually/ever used with a softPBX? How would this be connected?

  5. Internet based
    Do any telcos provide digital trunks over the internet? You can get x numbers and x lines that you can connect to your softPBX. I know that a lot of VoIP providers offer this, but I was wondering if they can be bypassed in order to get one less potential problem in the connection.

Are there any other typical connections used by telco’s to provide phone lines?

I have seen DID been “used” to describe a few different services. My understanding is that it is supposed to be an extra number on your existing lines which you can then program your PBX to handle differently than your main number. Is that right?

I am happy to do my own research, so if anyone has a good recommendation of book that covers this clearly, please let me know. There are so many books on this, I have read through quite a few but none were as clear as I’d hoped.

Thanks for taking the time to read this and responding.

Analog trunks are delivered using one pair of wires per line. One line is usually delivered in one RJ-11 jack. Two lines may be delivered using two RJ-11 or one RJ-14 jack. There are lots of other arrangements, with up to 25 lines delivered using RJ-48X (25 pairs appearing on a punch-down block with an amphenol 50-pin connector on it). Look up “registered jacks” online.

Digital trunks may be ISDN Basic Rate (two voice lines) or Primary Rate (up to 23 voice lines). They may be T-1, non-ISDN (24 lines).

VoIP trunks may be SIP or, less frequently, IAX protocol sent over the public Internet, or a private IP-based network. Look up “SIP origination” for inbound calls from the PSTN (public switched telephone network), or “SIP termination” for calls to the PSTN. DID (direct inward dialing) service refers to the provision of dialable PSTN-compatible numbers (like 212-345-6789), and is associated with SIP origination service. SIP origination providers offer prices for DID’s and channels. Each DID provides one dialable number, each channel carries one call. The number of DID’s and the number of channels need not be the same.