sounds like electrical noise from the inverter switching is getting onto the phone line and drowning out enough of the DSL signal to keep the modem from communicating with the DSLAM.
Things to look for are:
- ground loops: Is there another ground rod (or a grounding wire built into the foundation) hooked into the system somewhere? All the building grounding should be a "tree" with no loops in it. The inverter ground post should tie to the same grounding system, preferably close to where any other wiring from the RE system bonds (to minimize the length of the current path for harmonics from capacitive coupling to the case and then back to the other electronics in the inverter through the ground system - forming an inductive loop that can magnetically couple to other things). Hooking it to a separate ground rod puts this current into the ground around your house, causing different voltages to show up on all the ground rods if you have THREE or more separate grounds. Your gas piping, water piping, and electrical systems should all be tied - already - to a single grounding system. (There should also be a ground at the surge protector where the outside phone line attaches to the inside wiring, and this should be connected to the rest of the building grounding by the same rules: tree, not loop.)
- Ground fault in the phone drop wiring. (Does the phone sound noisy?) If you have such a fault, it will be "unbalanced", i.e. voltage on it will couple into the phone line signal path (which is a "balanced" differential signal - one wire goes + as the other goes - - to cancel out inductive and capacitive coupling to the wire). If you have a ground fault, injecting harmonic signals into the ground by tying your inverter case to a DIFFERENT ground than the rest of the RE system will inject them into the phone line. They might be too high frequency to hear - but that's right in the range where the DSL signal is.
I'd bet it's the combination of ground currents and a phone line ground fault.
If so you should get them BOTH fixed. You'll have to fix the grounding system. The tellco will have to fix the ground fault if it's outside the house - and (at least in the US) it's your responsibility inside the house and they'll charge you if it's there.
Where are you located?
Presuming your tellco works the same way as the US types, try this:
- with your ground disconnected (so the DSL modem works as is): Take the DSL modem out to the "demarcation point". Open it and see if it has a modular jack and plug on a little loop of cord inside. If it does, unplug the modular plug and plug the modem into the jack - thus disconnecting the house wiring and hooking the modem to the outside line directly. Power it up and see if you get the green light after the normal sync-up sequence. (You won't need to hook up your computer.) If it works, go to step 2. If it doesn't, you may have a DSL modem that uses the "line 2" connections on the modular jack rather than line 1. Get an adapter and try again. Once you get your sync-up go to step 2.
- Hook your inverter back to the ground rod in the way that made the DSL modem fail. If it flakes out while connected directly to the line there's a ground fault on the tellco's side (or in the modem). (You can eliminate the modem by powering it by batteries directly or a portable battery/small inverter combo and trying again. Flakeout with isolated power is DEFINITELY the phone line.) Call repair service. (In the US they can usually test the line using automatic equipment and will dispatch a crew to fix it for free.) If it doesn't flake out your phone line ground fault is on the wiring INSIDE the house- or something plugged into it - and you should hunt it down yourself. (Try the same test with the modem plugged into its normal location and all the other telephone instruments - fax machine, etc - unplugged to determine whether it's wiring or a device.)
Where are you located?
