Whether the source of the problem is external or internal will determine how you’ll need to approach a fix. Normally, it’s your phone provider’s responsibility to resolve external business phone system issues. So if that’s what’s happening, all you really need to do is get in contact with your provider. On the other hand, if the issue is internal, it will likely be your responsibility to handle the problem.
One way to tell the difference between external and internal issues is if every phone in the office is affected by connectivity troubles. This can include a lack of a dial tone, dropped calls, weird noises, or static. If this is the case, then the issue is more likely to be external. If only a few phones are experiencing technical difficulties, then the problem is probably internal, and you’ll need to call a technician or have your IT department look into the matter.