Delay to ring - External IP - Internet Speed

Kind of lost here.
Not an expert, but trying to get in there.
Questions are:

  1. When dialing the extension I use for Voice Mail Messages sometimes (not always) takes 20 to 60 seconds to star ringing. This also happens when calling from the outside to one of the lines.

  2. I am not using the box from the outside, do I need an external IP for the box?

  3. I am using comcast business as a ISP. When I start the gateway it gives me 14Mb down and 5Mb up. After some hours it drops to 1Mb down and 0,4 up.
    Comstast made some test and they say all is ok in their network. So I am supposing I have some issue but can’t find it where.

My installation is very simple. The only server runing is the Linux/Pbx with no file server and the computers don’t use internet (just mine). The lines are voip and no issues with them so far.
I understan this may be a too open question since all the technicalities envolved in this. Maybe someone can iluminate me where to search or what basic test can I run to detect issues.

Thanks in advance.

How do you access your VM messages - *97 or *98 ?
What do your incoming external lines connect to, a ring group ? IVR ? single phone ?
Your comcast connection sounds dodgy - do they throttle it or does something on your network use all your bandwidth ?

VM: in my extension I use *97, if I want to check other VM then I use *98 and the sys asks for the extension number and pass.

Lines: the sequence is:
Inbound -> IVR (listens to extension or) -> Time condition (close:to VM, Open: to Queue) Note: I have to use this setting b/c I have agents receiving incomming calls after hours)

ISP: tests always gives 14/4 (when normal). I am not sure what are (if) they doing, but technical support have been here for any times and they coulnd’t find any issues (I couldn’t reproduce the problem when they are here (Murphy’s law???)
There are no devices using BW here exept my computer and the voip lines.