There are several reasons why you might chose to keep your landline. First, and foremost is call quality with a landline you are almost certain to have good call quality. While with a cellphone there is a chance that your location has areas where calls can drop or not come through at all.
Also, depending on your location there maybe safety reasons why you need a landline, such as fire alarm, elevator phones or security systems.
With a landline or another unified system such as voice over internet protocol (voip) you can set-up a incoming message, voicemail messaging and directory service that is cohesive and onbrand. You can also easily transfer calls from extensions, and retrieve messages. However, if each employee has a cellphone there will not be a cohesive feel to their voicemail messages. Plus, it can be harder to forward messages from employee to employee.
Next, cost is a consideration depending on the type of cell phone packages you select and the number of employees it can be expensive to outfit each employee with a phone. Plus, you most likely will have to assign an employee to manage the cellphones i.e. issuing new phones, collecting phones from outgoing staff, repair/replacing for the inevitable lost or destroyed phone.
Plus, fundraising calls can contain sensitive financial information, that is perhaps best discussed in a quiet office, not the local coffee house.
The 24/7 aspect of cellphones could also be an issue for your organization, do you expect your employees to answer their company issued cell phones even on off hours? If work/life balance is an important cornerstone to your organization, you will need to institute a policy on phone use during standard non-business hours.