The number of fixed phone line customers continues to decline steadily as many domestic or small business users switch to mobile services. With the high upfront fixed cost and monthly line rental charges of a fixed line service, it can be more cost effective and practical for these lower traffic customers to choose mobile plans which generally have the same, or cheaper, advertised per minute call rates. It can also be easier for operators themselves and some larger businesses now use fixed PABX (private automatic branch exchange) systems in their offices that use mobile SIMs in each desktop handset to avoid the difficulties in installing a whole new wired system.