Usually, these phone shave an LCD display screen that will show you the caller ID on inbound calls. You can control multiple calls using a separate button for each call. For example, if you get multiple calls, each caller's number will be displayed. You then can either put the first call on hold and answer the second call or the second call will be transferred to voice mail. Conference calling is also supported by these telephones.
Prices range from several hundred dollars per telephone and higher, depending on the features.
By the way, if you purchase an ISDN telephone with an integrated NT1 interface, you can only put one telephone on the circuit. A better solution is to purchase the NT1 interface separately and then you can hook up to 8 telephones or TE1 devices to the ISDN circuit. These devices are ISDN telephones with a data port to allow PC access to your ISDN link. You can now transport voice and data simultaneously over your single ISDN circuit. So, if you are ona voice call and need to transfer a data file, send a fax or access the Internet at the same time, you have that capability.