In the US a typical analog line from the phone company would be -45vdc to -53vdc(usually 48vdc), a line from a PBX or the Grandstream would be 21vdc to 27.5vdc (usually 24vdc)....(and before any one post to tell me I'm wrong there there are different line cards in phone system's that supply more or less voltage)
Ring voltage would be
90VAC at 20 cycles per second, with a true sine wave. A message waiting light like the 2400 used was 90 to 140VDC (usually 90vdc) on the line when the phone is on hook. Since the message waiting light uses DC voltage it will not cause the phone to ring, but it may ding once as the voltage is applied. A majority of phone companies in the US use Studder DIal tone to signal a user that they have voicemail. Studder dial tone is where the dial tone you hear instead of being a solid tone is broken. Studder dial is slowly starting to fade away, because for users to know if they have a message they have to lift the handset on there phone. With only a few users it is not a problem but with thousands of users checking for voicemail it causes the phone company's switch to slow dial and require more register sender cards (cards that supply dial tone). The grandstream like most newer devices coming on the market today use FSK signalling for caller ID and message waiting. FSK signalling is a lot like caller ID on your home phone where the phone company sends a momentary data burst down the line while the phone is on the hook. This data burst will tell the phone to turn on a message waiting light.
I found this on Grandstream'ss website.
Message indicator is a special on-hook caller ID type message that enables and disables the message waiting light on certain phones. GXW4008 has this feature enabled by default. However, certain phones (rare) that do not support it may mistakenly treat this CID signal as an incoming call. A configuration option is needed to turn on MWI in this case.
So I was wrong to a point the Grandstream does not support the Message waiting lights on your current phones but you could replace the phones with newer sets that support FSK and it would work.
The voltage the Grandstream uses to ring the phones and give dial tone comes from the power cube supplied with the unit. The Grandstream only supports 1 device per port so it does not need a large power supply like the phone system uses.
Hope this helps 