The "target phone is ringing", also known as 'ringback' signal is sent in the direction target->caller.
There are two basic types of such message in SIP world.
The first one (type 180) is just message - "target is ringing". This type of signal carry no particular tone. It's up to caller's PBX (or even caller phone) to play audible signal.
The second one (type 183) is message "I will provide progress tones to caller by self". This type of signal starts one-way audio channel to the caller, progress tones, including the ringback one, can be passed trough it.
The first type is preferred. The ringback signal is generated at caller side - so the signal known to caller is generated (moreover no bandwidth is consumed to carry it). The second type is mostly used to carry nonstandard signal, spoken messages or so.
To make things complex, any PBX on the path may consider to change message of 180 type to 183 and vice versa.
PAP2T responds 180 all the times. According your description, it seems someone considered to claim "I will play audio signals", but then it play no signal. Or someone considered cut of the audio (not every operator allow early audio to pass thru it's network) but didn't reverted 183 message to 180 message.
It may be your VoIP provider, it's upstream provider, the provider carrying the call to PSTN, an transit PSTN operator and even originating PSTN operator on calling side.
No one will disclose call path to you - it's considered trade secret. And even if you will know it, you are not eligible to call all those operators asking them to do something. You have neither business relationship to them, nor you can provide call details (known to "previous-in-path" operator only) to them.
As I has claimed in previous comment - if calling user is affected by an issue, then he should report it to their operator. Such operator will analyze the issue and ask next-hop operator for help if necessary. Same procedure will be repeated until an operator causing the issue will be reached.
Well, you may complain to your operator, but he may be unable to help you.
