Local Phone numbers: Advantages & Disadvantages of VoIP
It lists the disadvantages of VoIP compared to conventional (analogue) telephony. In this case we are talking not only about asterisk, but about any VoIP systems: both paid and free.
High cost of equipment (usually not the PBX itself, although some digital/program PBXs are very, very expensive).
An IP PBX needs a computer (it can be part of the PBX, but it is still a computer), which means that disk failures and other components may occur.
As a rule, the complexity (and therefore expensive) of the initial setup
In any case (and in the case of connecting the PBX to the Internet - increases many times) there is a risk of hacking the PBX, in this case the amount of payment for telephony can be tens and hundreds of thousands of dollars (and this is no exaggeration). You can get landline phone number here.
As a rule, complexity of support of telephony (addition of subscribers, change/addition of operators of telephony, etc.), demanding presence of the qualified expert (any telephonist / the computer operator here will not approach).
At individual configuration setting (which is the rule) PBX programming is required, as a result (as with any program) there may be failures, errors, etc., which may result in "wrong" calls (and other problems), which at best creates confusion, and at worst may spoil the image of the company and / or lead to additional costs for telephony.
Having many features in a digital PBX, many of which are enabled by default, is not always good. For example, the PBX can record conversations, as well as listen to any telephone conversations in real time (and much more). Employees who know how to use such options can theoretically use them (unless you provide additional protection or disable such features of the PBX).
For telephone communication, the local computer network (and the Internet, when connected to SIP providers) is used, so it is possible to break the connection, poor sound quality, etc. with high network load and/or slow/loaded Internet connection.
As with analogue telephony, there may be failures/breakdowns/glitches of voip equipment (both ip phones and voip/gsm gateways, adapters, etc.), but since voip equipment is more complex, the risk of its failure is higher.
Faxes. Although some managers are trying to assure that faxes are transmitted better than analogue phones via ip telephony, this is not the case. In an ideal situation, ip faxes are transmitted as well as analogue faxes. Any optimization/compression of telephone traffic results in impaired or even completely impossible fax transmission.
Since a computer network is used for telephone communication, it is possible (as well as to intercept passwords) to overhear telephone conversations (regardless of the PBX settings), "peek" at which numbers call subscribers, etc.
And finally, in fact, VoIP is not always the cheapest solution. In some cases (for example, for mobile calls within the Russian network of one operator) the price of a call may be lower than through VoIP (the cost of a call may even be zero, depending on the tariff and operator). So in each case it is necessary to make an individual decision about VoIP application.