The Improved Mobile Telephone Service (IMTS) is a pre-cellular VHF/UHF radio system that links to the PSTN. IMTS had originally only sixteen channels and were prone to network congestion and interference by others "keying up" their radios while a converstaion was already occurring on a channel. Some IMTS radio phones have their own DTMF pad while others required the local operator to put the call through. Many IMTS frequencies are now used for local paging services. They are only found in some parts of rural North America as of late 2004, having been replaced in most areas by cellular service in the 1980s.