FaxFinder - Modem init string options when having trouble faxing from behind PBX

For analog FaxFinder product line.

Debug logs may show that the fax modems are missing low speed V.21 or not seeing high speed at various times.

This may be caused by the default signal debounce settings within the modem.
The default settings were designed for use on analog PSTN circuits but now that many FaxFinders are being put behind PBXs, these parameters may need to be adjusted for the PBX environment.

Adding the following string to each modem initialization page may help.

- Notes on the Fax S registers:

+FS1: This controls V.21 detection timer (1 mS increments).
+FS2: This controls the number of times for a valid V.21 signal.
Combined, +FS1 and +FS2 control how the modem detects V.21 while looking for either high speed or low speed signal.

+FS10: Time to wait before re-initializing V.21 to receive frame.

+FS18: This controls high speed detection timer (10 mS increments).
+FS19: This controls the number of times for a valid high speed carrier.
Combined, +FS18 and +FS19 work together to determine the threshold when the modem sees carrier bouncing and whether it should stay in high speed or look for low speed.

- Other recommendations that might help with various faxing issues:

1. Add AT+FRY=2 or ;+FRY=2 to increase the number of retransmits during an ECM fax. +FRY=2 will perform 8 retransmits, +FRY=3 will perform 12 retransmits and default is +FRY=1 which is 4 retransmits.

2. Setting ATS91=8 or S91=8 will increase the modem transmit (TX) level. This will help if the remote end is having trouble receiving from the FaxFinder.

3. Adding AT+FCQ=0,0 or ;+FCQ=0,0 will turn off Quality Checking to prevent RTN frames on non ECM faxes.

4. Adding AT+FS8=30 or ;+FS8=30 will extend the T2 time to wait after receiving a page.