VoIP is dependent on bandwidth. Even the best of the best ISP (Internet Service Provider) experience technical issues. Since VoIP uses your Internet connection, it is bound to be affected. If your Internet is down, you will not be able to make or receive a call. If your connection is exceptionally slow, then expect low voice quality. A user might also encounter problems if he makes or receives a call while streaming videos or audio, or while downloading a large amount of data. If bandwidth is the problem, then it might be time to change your provider. Search for a more reliable and stable ISP and don't forget to ask your friends for referrals.
Voice quality issues. This has been a problem attributed to VoIP calls ever since its birth. Static noise, echoes, and delays are a common complaint of users. There are a number of factors that affects VoIP's voice quality: internet connection, the hardware in use, the VoIP provider, distance of call, and many more. It is best to try out the service before committing to it. Also, try different combinations of hardware until you get the results that you want.
VoIP needs power. Unlike traditional phones, VoIP needs electricity to work. You have to plug in your modem, ATA, and other VoIP hardware for you to be able to make and receive a call. You can not use your phone if there is a power interruption. Emergency power generators might be a little far-fetched for a regular home to have, but if you are running a business then you should probably get one.
Emergency calls. If you use traditional phones to dial 911, it will be diverted to a call center nearest to you where the operator has the ability to see where exactly you are calling from. Using VoIP, there is no way to trace where your call is coming from. In addition, if the emergency involves a power interruption then you will not be able to make a VoIP call. There is a new service called E911, which a lot of VoIP providers are offering. It is for your best interest to ask your provider regarding this and other emergency calling issues.
To be on the safe side, having a traditional phone aside from the VoIP phone is a good idea. It will also help if you could write down individual phone numbers of emergency facilities (local police, fire station, hospitals, etc.) and post it near your phone.
Keep in mind that these are just minor inconveniences and every passing day VoIP is advancing. Its advantages and benefits are too many to pass up. Soon it will replace traditional phones as the communication technology of choice.