I new to the Hosted PBX and VoIP systems. Currently the company I work for has an old T1 phone system they are looking to move away from. We have Skype for Business and Office 365. Basically my question is, to use this system to make phone calls outside of our office will we need a ITSP? Microsoft claims to provide Unified Communications, which to me would sound like a hosted PBX. If anyone could fill me in on this I would greatly appreciate it.
Office 365 has Skype PBX/PSTN in preview, not in production today 2015-10-14. There are other hosted providers you could go with to get a dial tone in S4B, or you could build some voice infrastructure on-premises. ServerFault doesn't allow recommendations, and I couldn't make one without knowing what your business needs are.