First rule of asking for help: Post enough information so someone has a picture of your setup.
You have fallen into the trap of posting as little info as possible
for example: How many providers are connected to your obi?
What are the names of the providers, and what SP are each one connected to?
<if you tell me you have a copper telephone provider on SP1, then ringing once, is a common problem associated with a ground on your line>
Have you had a lightning strike /power surge / brown out etc etc recently?
stuff like that
In regards to Odd thing #2,
[First rule of trouble shooting, if you suspect hardware problems, swap out equipment for a known good piece of equipment]
Connect a known good working regular telephone,(not a cordless phone preferably)
and see if that makes any difference.
You probably do not have a spare Obi, but you should have a regular telephone to test, or be able to borrow one.