My conference call members can't connect

I was using the original free conference call number with user id for a weekly call at 6:30 pm on Thursdays, but some of my team had problems getting through; the number was busy all the time and they had to keep redialing. I switched to one number to make it easier since adding the user id was also a problem, but now it is worse. They are not even getting a busy signal. Is there an alternative number I can give them that they can use instead any time this happens, or are backup numbers only valid for one call? Would it be better if the time of the call was changed?