How to capture RTP packets?

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I am trying to find the clock drift information for each of the machines (e.g. my work laptop, my personal laptop, cell phone, etc.) using my router/Internet. Based on other posts [1][2], I am led to believe clock drift information can be found in RTP packets. However, after filtering for "rtp" packets in Wireshark, I find none.

According to [3], UDP packets can be converted into RTP packets. So, I tried enabling the "rtp_udp" protocol. I still only see UDP and no RTP packets. I also tried selecting a UDP packet then: Analyze -> Decode As -> RTP (in the Current column). It seems like the UDP packet is converted, but I don't see clock drift information.