During a call between an analog telephone line and Tel-14 interface the line is being dropped randomly.
It is worth noting the sound you here when the line is dropped to help you identify which device is dropping the line. The Tel-14 always ends a call cleanly with no pop or crackle. CPC generators may drop the line more aggressively with a loud pop.
If you believe it to be the CPC dropping the line it maybe that the Auto Gain on the Tel-14 has increased the audio level too high for the CPC causing it to interpret the line audio as tones or signals. 
Turn off the Auto Gain on the Tel-14 and adjust the Send Receive volume manually. Start by turning them right down and then initialise a call in the normal fashion. If the line stays up without being dropped increase the Send Receive volume to a point where you are happy with the audio level but have not yet caused the line to be dropped.
Also perform an Auto Nulling procedure on the line making sure that the CPC is in circuit during this process.