T1 lines are full duplex. They are synchronous lines running at 1.544 Mbps. You actually get two separate pairs, one pair for each line. This is also known as 4 wire transmission. A regular phone line is only a single pair of wires. Each pair of a T1 line has a constant signal present. Even when there is no data or payload being transmitted, there is always a framing signal running at 8 Kbps on each pair. What you have available is 24 each of 64K channels or 1.536 Mbps available for your data. It matters not whether you actually send or receive something. The channels are sitting there, idling, always ready for use. Synchronous systems have to do this or they lose track of where they are.
This standard for the T-carrier system as it is known in the telco world was established to support digital telephone transmission. Each channel can handle one phone conservation with talk on the transmit channel and listen on the receive channel. Since it is a digital carrier you can easily substitute Internet data instead of digitized phone audio or use all the channels for a single broadband Internet connection.
Full duplex is important for telephony. You need to be able to talk and listen at the same time. Get into a heated telephone conversation and you'll hear this capability in action. Many speakerphones are not full duplex, though. In order to suppress echoes, whoever is talking loudest tends to cut off the other side of the conversation. This is called half-duplex. Half duplex means that each party can talk, but only one at a time. CB and Ham radio sets are half-duplex. You push a button when you want to talk.
There's another standard called simplex that is one way transmission only. Telemetry from a weather balloon or satellite TV broadcasts use the simplex mode.
Full duplex operation may or may not be critical to what you are doing. For phone calls, including standard phones on a PBX system or VoIP telephony, full duplex is critical if you want something that works like a real telephone should. The need seems less apparent when you are entering web site addresses on a browser and then waiting for the pages to download.
Another use of T1 lines that highlights the capability of full duplex is radio broadcast studio to transmitter links. Program audio from the studio is digitized and sent to the transmitter location miles away. That channel is usually fully occupied all the time. The other channel, sometimes called the backhaul channel, can be sending a different program from a satellite dish at the transmitter plant back to the studio. With full duplex, two programs going in opposite directions can use the same T1 line without interference.
Changes in the way Ethernet networks are deployed has muddied the waters about what full duplex really means. The original Ethernet standard allowed multiple senders to transmit at the same time and their packets would collide, effectively lowering the 10 Mbps bandwidth of the network. In practice, it was a half duplex system. Now that most networks are using switches instead of hubs, those collisions are eliminated, allowing for a full duplex system with 10 Mbps transmit and receive. Fast Ethernet using switches is good for 100 Mbps. Gigabit Ethernet is, well, a Gigabit per second.
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