Linking Calendars

I am a new user to 365 and have been using an exchange server to date. 

 I have one calendar which is shared between 2 users who each see it in outlook and on their iPhones.  If we use the calendar in 365 this will not synchronise with outlook or the phones.  If we continue to use the outlook calendars then these will not be synchronised with each other.  Is there any way of redeeming this problem?