How do I include PSTN callers in a Meetme Conference

I’ve been racking my brains (not very sharp these days) on how to include PSTN callers in a conference. The meeting rooms have been set up and the conference number is one of my SIP extensions. Since PSTN callers cannot dial this number I am trying the following:

In extensions.conf

exten =>0021671833224,1,Goto(Conferences,${EXTEN:5},1)

exten => 5010,1,MeetMe(5010)
exten => 71833224,1,MeetMe(5010)



conf =>5010,717

However when I try dialing the PSTN line I get our normal IVR menu. Is there a way of making it possible for people to join a conference over a PSTN line?