I'm going out on a limb here, and guessing it's a Canon imageClass MF4150... If not, please disregard this solution.
The advanced manual can be viewed / downloaded / printed here, directly from Canon:http://gdlp01.c-wss.com/gds/0900006459/imageCLASS_MF4150_Advanced_Guide_EN.pdf
To stop it from interfering with the telephone conversation:Issue:
Fax interrupts telephone conversations in answering machine mode (AnsMode)
Setting the Receive Mode - ICMF 4150
Setting the Receive Mode
- Press <Additional Functions>.
- Use <> or <> to select [5 FAX SETTINGS], then press <OK>.
- Use <> or <> to select [3 RX SETTINGS], then press <OK>.
- Use <> or <> to select [3 RX MODE], then press <OK>.
Use <> or <> to select the desired receive mode, then press <OK>.
For [AnsMode], make sure an answering machine is connected to the machine.
If [AnsMode], [FAX/TEL], or [DRPD] is selected, you can select some settings as below:
[AnsMode CHK TIME] (select time from 10 to 30 seconds for the answering
machine to start playing outgoing message after receiving a call)
--- [FAX DETECT TIME] (select the duration from 10 to 70 seconds for the fax to detect)
--- [RING START TIME] (select the time from 0 to 30 seconds for the phone to start ringing after receiving a call)
--- [F/T RING TIME] (select the duration of ringing from 10 to 45 seconds)
--- [F/T SWITCH ACTION] (select the action the machine will take after the ringing ends, Receive or Disconnect)
--- [NORMAL RING]
--- [DOUBLE RING]
--- [OTHER RING TYPE]
- Press <Stop/Reset> to return to the standby display.