Electronic Alarms and Home Security Devices - PC1565 V2.3 problem with third party cellular link

I have a PC1565 V2.3 connected to a third party cellular link board. The only wiring between the two is through the TIP-RING. When the PC1565 wants to send an alarm by telephone line, the third party board activates a cellular call. When cellular board is not connected to PC1565 it works fine (cellular link is active), but when I connect it to PC1565, the cellular link works fine for some hours or days. Both systema have a separate 12 VDC power supply. Could there be a grounding problem?
Thank you.

It is generally recommended that when alarm devices with seperate 12vdc power are connected together, you tie the grounds together. Some alarm systems and add-ons have varying degrees of sensitivity to this effect, so it's worth a shot.

Thank you for your recommendation. When you say that I should tie the grounds, do you mean the negative of the power supplies? Only one power supply has a grounding terminal (actually it is the alarm board) but the second power supply does not have grounding terminals. Is it OK if I connect both negative terminals?

Yes, tie the two negatives (and only the negatives) together. Make sure that cell dialer's power supply is actually DC, some of them use 16vac power packs.