The string/code should be independent of whether you transfer to mobiles or land lines. It will be different based on the phone provider you use. I know that the default ( !, ) works in many cases, and with most PBX systems.. but in other cases (such as the Vonage phone service) it fails. I'm not sure how you determine the correct transfer code for your telco provider, that you might have to ask them (the provider) or wait here for someone else to respond.
Edit: Converting to digital is not necessary, although if you converted the lines to VoIP using an FXO adaptor, you could transfer calls using a VoIP provider. This would not only be cheaper (call cost wise), but would solve the problem with not knowing the string code. The only disadvantage is setting up an FXO adaptor is a bit complicated and if you aren't experienced in that area it may be somewhat difficult to get your head around.