The following is based on my experiences with VoIP over the past few years. Some people can buy a VoIP adapter, or obtain one from a VoIP provider, and make it work the first try. Others have all manner of issues, and they may not be due to the actions of your VoIP service provider. This is not intended to be a comprehensive installation guide, but rather a list of pitfalls you may encounter and tips on avoiding them.
1. Cable Internet works better than DSL. The problem here is generally the DSL modems and they way they are configured. Telephone companies tend to buy the cheapest DSL modems they can get, but typically these DSL modems contain a really crappy router that is enabled by default (you don’t realize it because there’s typically only one output port, but often there really is a very basic router in there). Then the customer plugs in their own router and presto, you have a situation known as “Double-NAT” (NAT=Network Address Translation). That is NOT good for VoIP. I’ve seen situations where a VoIP adapter won’t work at all unless it’s plugged into the DSL router directly (which typically leaves no place to plug in a computer) and situations where with a dual-line adapter, it’s impossible to get both lines to work simultaneously. One possible way to resolve the problem is to put the DSL modem into what’s called “bridge mode”, which in effect disables its internal router, but most phone companies don’t tell people how to do that, and it’s a process that scares some users (“But what if my Internet stops working altogether?” And that definitely can happen if you do it wrong!). One other problem with DSL: While many cable providers tend to let you keep the same IP address for months if not years, DSL providers seem to change users’ IP addresses much more frequently, and that can cause firewall issues (see point #4).
2. Some routers are VoIP friendly and some are decidedly not. So your old router either bites the dust, or won’t keep up with the latest speed upgrade from your broadband provider, and you head online or to a store for a new one. Chances are it doesn’t cross your mind, “Will my VoIP service still work?”, but it should. There are certain brands of routers that are utter garbage as far as I’m concerned, and then there are other brands that are much better. For some odd reason, the newer the router, the more likely it is to cause certain types of issues. One unfortunate thing is that most consumer-grade routers today seem to come with wireless capability even if you don’t need it, and for some reason those newer wireless routers seem to be more prone than usual to cause problems. In other cases, routers come with specific settings that are supposed to help VoIP, but don’t. If you control the VoIP server and endpoints, one thing you can try is running your SIP traffic on ports not normally used for SIP (typically 5060-5061) and if that suddenly works, it may be that your router is trying to manage SIP traffic and doing it wrong (some routers may have a check box that you can check or uncheck to turn this feature on or off). Before buying a router, make sure that if it messes up your VoIP connections you can either return it, or reflash it with third-party firmware (Tomato, DD-WRT, etc., assuming you feel confident in your ability to do this. I assume no responsibility if you “brick” a router because you don’t know what you’re doing!).
3. Some VoIP adapters and equipment will work in certain situations but not others. One thing about the venerable Sipura SPA and Linksys PAP series VoIP adapters is that they will work in many situations where other, cheaper models won’t. Early on we used a few cheap adapters, and while these were always a bit of a pain to get working, once set up they tended to be pretty reliable — until the newest crop of routers started coming down the pike! So the typical story is, user buys new router, old adapter (that has worked great for years) won’t work, user buys new unlocked Linksys PAP2, their VoIP works again (obviously I am talking a situation where the user provides their own adapter). Bottom line is, some combinations of equipment don’t play well together (and that can include equipment provided by your ISP and/or your VoIP service provider).
4. Firewalls can be an issue. One thing I suggest if you are having issues is to temporarily disable any firewall you may have if you are having an issue getting a VoIP device to register, with the caveat that I primarily mean at the server end and not at the user end (which is not to say that a user firewall can’t be a problem, but you don’t want users running with no firewall protection). If you are able to do so, drop the firewall for a few seconds and have the user power cycle their router and VoIP adapter, bringing the router up first, then the VoIP adapter. If they can suddenly register, then you may have a too-strict firewall policy, or you may simply need to punch a hole in your firewall for traffic from their IP address. Obviously you want to minimize the time your firewall is offline, and you don’t want to do it when you know you’re under attack. But if you have used good strong passwords on everything in your system that allows outside access, and shut off all services that provide external access and that don’t need to be running, you can minimize your exposure. There is one particular VoIP software package out there that prides itself on its security, but it’s so secure that it causes problems for remote endpoints, particular ones on frequently changing IP addresses (in other words, some DSL users). I do NOT advise running a server without a firewall.
5. Sometimes, you just need to wait. I’ve seen it happen where a user gets a new VoIP adapter, configures it with the same settings as their old one, then disconnects the old one — and the new one won’t work. Then they go away for an hour or two and suddenly the new one miraculously starts working. Why? Without getting too technical (or straying into territory where I don’t know what I’m talking about), I believe it has to do with “lease times” on connections in the router, or possibly even at the server. If you really need to get it working now, try power cycling both the router and the VoIP adapter, bringing the router up first, then the VoIP adapter.
6. Power supplies fail in strange ways. One final tip that I’ve discovered is that sometimes power supplies on VoIP adapters partially fail (or the cord that connects the power supply and the adapter might go bad). When that happens, it’s not always complete failure — the adapter may become noisy, or may exhibit all sorts of strange behavior (such as connecting, then dropping the connection for no apparent reason). I just saw a malfunctioning Linksys PAP2 brought back to life by replacing a bad power supply, and it’s not the first time.
7. If all else fails, you could try using a Linksys SPA-2102, a Grandstream HT-502, or similar VoIP adapter that includes a built-in router (I tend to prefer Sipura/Linksys devices, but understand that the Grandstream units will support those old antique rotary dial phones that most kids have never seen, except in the movies and on old TV shows). You’d connect the output of your DSL or cable modem to the INTERNET or WAN port of this unit, then connect your computer, a plain old dumb switch, or possibly your existing router to the ETHERNET or LAN port on the VoIP adapter (the latter combination possibly creating a triple-NAT situation for downstream devices, if you have DSL). Remember that if you do this, you’re limited to whatever connection speed the VoIP adapter will pass through, usually 10/100Base-T, so you probably don’t want to do this if you have cable Internet service or fiber to the home! The only advantage of doing this is that if you have a router that doesn’t play well with your current VoIP adapter, you can connect one of the VoIP adapters with a built-in router directly to your cable or DSL modem and get your existing router out of the picture as far as your VoIP is concerned. I’d say it’s probably a better idea to get rid of the problematic router (if you can’t reconfigure it or download new firmware to make it work), but if you don’t know what you’re doing you might just buy another router with the same problem. (EDIT: Another possibility might be a Linksys RT31P2-NA router that includes VoIP capabilities, if you can still find an unlocked one for sale — I’ve never used one of those, so can’t say for sure how well it might work in this situation).
Do you have any other tips for making VoIP work, or any horror stories you’d like to share? The comments are open!