I have a strange problem when trying to power the base station of a DECT phone with a BUCK module wired to a 12VDC battery charged by a solar controller.  

The base station of the DECT phone lights up, the paging (find phone) function works, but I get no dial tone on the handset.  The phone wire end is plugged into a VOIP box powered by the same battery.  If I use the same setup with the base station 240v adaptor plugged into an inverter (even a modified sine wave one), it works fine.  If I supply power via a BUCK module to the base station using a standalone 12v battery, it works.  A corded phone plugged directly to the VOIP box also works.

I have tried different phones: a Panasonic with 5.5v, and a Uniden with 9v.  The result is the same.  Thinking it might have been a voltage problem, I varied the voltage up or down slightly to no avail.  I prefer to run the phone through the BUCK module rather than using an inverter all the time.

Thinking that there may have been some grounding incompatibility, I have tried the following:

1. Powering the BUCK module directly from the 12V battery, rather than through the output side of the solar controller.  No difference.

2. Powering the BUCK module through a completely separate solar system directly connected to the 12V battery.  It worked initially, but when I noticed that the controller wasn't charging the battery; reset the controller, the DECT phone stopped working.

The battery voltage varied from 12.7v to 14.4v during different stages of charging.  The output of the BUCK module appears steady.  It appears as if connecting the battery to the solar controller "dirties" the supply current.  One PV system has a MPPT controller.  The other one has PWM.  Different brand controllers, no difference.

The next thing I'd try is to put a bridge rectifier and a capacitor in front or behind the BUCK module.  I'd use a choke too but I wouldn't know what kind to use.

What gives?