Conducting research using random digit dial (RDD) landline numbers has for decades been the staple of the research industry. In recent years the effectiveness of this methodology has been in significant decline; first due to the withdrawl of electronic White Pages from public access in 2002, followed by a significant decline in landline installation at home (no longer necessary now that everyone has mobile phones).
The ACMA Communications report for 2011/2012 shows that only 22% of Australian adults who have a fixed line telephone or mobile most use fixed-line at home to communicate, meaning that even when people have a fixed line, it is usually not their primary method of phone communication. Furthermore, the incidence of having access to a fixed line telephone is low amongst younger adults. In June 2011 it was found that only 63% of 18-24 year olds (mostly those still living in their parental home) and 64% of 25-34 year olds claimed to have a fixed line telephone at home. These figures have been falling over the years, so are most likely much lower than this now. 
Research conducted by the Social Research Centre reveals that there are statistically significant variations in the populations reached by different telephone sampling methodologies. Specifically, those who were contacted over a mobile phone who didn’t have a landline showed a higher incidence of being male, in younger age groups, live in capital cities, born overseas, live in rental accommodation, and living in their neighbourhood for less than five years.  In addition, significant biases were observed in the sample contacted over landline, with landline sample showing lower levels of a variety of important variables including health issues, public transport usage and smoking and alcohol consumption. 
There are telephone number list providers out there that claim to include mobile numbers by region. These are ‘listed’ mobile numbers. That is, when someone obtains a new mobile number, it is default unlisted unless the owner requests that it is listed. Many mobile number providers don’t actively prompt for people to have it listed. Mobile numbers that are listed are highly likely to be home businesses (as these are the people who go out of their way to get their numbers listed), thereby skewing the ‘mobile population’ in the survey.
Using Random Digit Dial (RDD) with a mix of mobile and landline numbers would be viable to achieve representative samples. However, this will only work for national surveys, as mobile phone numbers are not assigned by region. Undertaking local area telephone surveys using RDD landlines or white pages phone numbers (even if listed mobiles are included) will miss large, and often critical chunks of the community.
It should be noted, however, that telephone surveys would still be viable if you are sampling a population where you have phone numbers for the entire population (eg. using a client list).
 Australian Communication and Media Authority (2012) Communications report 2010–11 series Report 2: Converging communications channels: Preferences and behaviours of Australian communications users, ACMA.
 Penney, D (2012), Second national dual-frame omnibus survey announced, www.srcentre.com.au, accessed 21 August 2013.
 Penney, D & Vickers, N (2012), Dual Frame Omnibus Survey: Technical and Methodological Summary Report, The Social Research Centre.