Short Answer
Just about everything.
Long Answer
As with most things: it depends on your situation. In general, just about everything in IT can be outsourced, whether it's infrastructure management, helpdesk support, phone systems, or even custom applications. What you should outsource, however, is really where the decision making comes in.
These decisions largely depends on two things:
1. The nature of your business
2. The size of your business
Considering the nature of your business can really help you answer what services should be outsourced:
For example, a doctor's office would probably be well suited to outsource all of their IT needs to a managed service provider ("MSP"). Their IT needs are fairly straightforward, changes to infrastructure are fairly infrequent, and they usually can't justify hiring multiple staff members to manage IT for them. A software company, however, would usually only want to outsource certain functions of their IT operations. Application servers and databases are probably best managed by an in-house cloud engineer. User workstation and phone management are definitely worth outsourcing, however, because managing these needs means hiring on a whole different breed of technical staff, not software developers.
The size of your business is the other big piece of the puzzle:
A small business with anywhere from 1-100 can almost never justify hiring their own IT staff. Not only does hiring an IT team mean management needs to know what to look for and how to manage them, but there's also usually not enough work to keep a full-time employee busy. As this number grows, however, it may make sense to bring on an overall IT manager - someone on-site daily to focus on the big picture initiatives. This doesn't mean they should do everything though, and that's exactly where IT outsourcing comes in. An outsourced team can run helpdesk operations, or even more specialized roles like network engineering, so the IT manager can be free to continue driving the larger tech pushes forward. Large enterprises can still outsource needs too, like large hardware migrations or specialty infrastructure consulting.
Conclusion
In general, just about everything in IT can be outsourced to some extent. What your business really needs to consider is what should you outsource. If you're not sure of the answer to this question, reach out to Cosmistack for a no strings attached consultation! Our primary goal is to educate and advise your business on the best steps forward for continued growth. We look forward to working with you!