Bit-rates
If one phone uses a high-bitrate codec, and the other a low-bitrate, the user with the high-bitrate will be able to hear the user with the low-bitrate, but possibly not the reverse.
Even if all phones used the same audio codec, packet retransmission and re-encoding are still required. When a user makes a call to a POTS phone, the audio is digitized into packets that are transmitted from the mobile device and are then decoded into 64 kbit/s PCM. This creates little extra noise, and if the mobile signal strength is sufficient, the user on the POTS line will be able to carry on a conversation.
Tandeming can be demonstrated using a personal computer, a microphone, and Apple's QuickTime Pro software, version 6.5 or above.
Read more about this topic: Tandem Signaling