Voice over IP
Voice over IP (VoIP) is a method that takes your speech, generally compresses it, and sends it via a TCP/IP network to another person who will decompress it and play the audio. This allows you to make a telephone call over the internet instead of using the traditional "landline" telephone system. Multiple locations in a major longer-term disaster (SHTF-TEOTWAWKI-PAW) situation and may be able to set up a Wifi link between the locations and they could use VoIP to utilize the same data link they have for voice traffic as well. This would enable multiple concurrent telephone calls over the single link, providing both ends have enough equipment on hand to accomplish this.